diff --git a/python/ray/__init__.py b/python/ray/__init__.py
index 1118313ae..8fb884ff8 100644
--- a/python/ray/__init__.py
+++ b/python/ray/__init__.py
@@ -78,27 +78,11 @@ from ray.profiling import profile  # noqa: E402
 from ray.state import (jobs, nodes, actors, objects, timeline,
                        object_transfer_timeline, cluster_resources,
                        available_resources)  # noqa: E402
-from ray.worker import (
-    LOCAL_MODE,
-    SCRIPT_MODE,
-    WORKER_MODE,
-    cancel,
-    connect,
-    disconnect,
-    get,
-    get_actor,
-    get_gpu_ids,
-    get_resource_ids,
-    get_webui_url,
-    init,
-    is_initialized,
-    put,
-    kill,
-    register_custom_serializer,
-    remote,
-    shutdown,
-    show_in_webui,
-    wait,
+from ray.worker import (  # noqa: F401
+    LOCAL_MODE, SCRIPT_MODE, WORKER_MODE, IO_WORKER_MODE, cancel, connect,
+    disconnect, get, get_actor, get_gpu_ids, get_resource_ids, get_webui_url,
+    init, is_initialized, put, kill, register_custom_serializer, remote,
+    shutdown, show_in_webui, wait,
 )  # noqa: E402
 import ray.internal  # noqa: E402
 import ray.projects  # noqa: E402
diff --git a/python/ray/_raylet.pyx b/python/ray/_raylet.pyx
index 5a52a22c1..b0bb90957 100644
--- a/python/ray/_raylet.pyx
+++ b/python/ray/_raylet.pyx
@@ -66,6 +66,7 @@ from ray.includes.common cimport (
     TASK_TYPE_ACTOR_TASK,
     WORKER_TYPE_WORKER,
     WORKER_TYPE_DRIVER,
+    WORKER_TYPE_IO_WORKER,
     PLACEMENT_STRATEGY_PACK,
     PLACEMENT_STRATEGY_SPREAD,
 )
@@ -90,6 +91,7 @@ from ray.includes.ray_config cimport RayConfig
 from ray.includes.global_state_accessor cimport CGlobalStateAccessor
 
 import ray
+from ray import external_storage
 from ray.async_compat import (
     sync_to_async, get_new_event_loop)
 import ray.memory_monitor as memory_monitor
@@ -590,6 +592,49 @@ cdef void gc_collect() nogil:
                     num_freed, end - start))
 
 
+cdef c_vector[c_string] spill_objects_handler(
+        const c_vector[CObjectID]& object_ids_to_spill) nogil:
+    cdef c_vector[c_string] return_urls
+    with gil:
+        object_refs = VectorToObjectRefs(object_ids_to_spill)
+        try:
+            urls = external_storage.spill_objects(object_refs)
+            for url in urls:
+                return_urls.push_back(url)
+        except Exception:
+            exception_str = (
+                "An unexpected internal error occurred while the IO worker "
+                "was spilling objects.")
+            logger.exception(exception_str)
+            ray.utils.push_error_to_driver(
+                ray.worker.global_worker,
+                "io_worker_spill_objects_error",
+                traceback.format_exc() + exception_str,
+                job_id=None)
+        return return_urls
+
+
+cdef void restore_spilled_objects_handler(
+        const c_vector[c_string]& object_urls) nogil:
+    with gil:
+        urls = []
+        size = object_urls.size()
+        for i in range(size):
+            urls.append(object_urls[i])
+        try:
+            external_storage.restore_spilled_objects(urls)
+        except Exception:
+            exception_str = (
+                "An unexpected internal error occurred while the IO worker "
+                "was restoring spilled objects.")
+            logger.exception(exception_str)
+            ray.utils.push_error_to_driver(
+                ray.worker.global_worker,
+                "io_worker_retore_spilled_objects_error",
+                traceback.format_exc() + exception_str,
+                job_id=None)
+
+
 # This function introduces ~2-7us of overhead per call (i.e., it can be called
 # up to hundreds of thousands of times per second).
 cdef void get_py_stack(c_string* stack_out) nogil:
@@ -650,17 +695,25 @@ cdef void terminate_asyncio_thread() nogil:
 
 cdef class CoreWorker:
 
-    def __cinit__(self, is_driver, store_socket, raylet_socket,
+    def __cinit__(self, worker_type, store_socket, raylet_socket,
                   JobID job_id, GcsClientOptions gcs_options, log_dir,
                   node_ip_address, node_manager_port, raylet_ip_address,
                   local_mode, driver_name, stdout_file, stderr_file,
                   serialized_job_config, metrics_agent_port):
-        self.is_driver = is_driver
         self.is_local_mode = local_mode
 
         cdef CCoreWorkerOptions options = CCoreWorkerOptions()
-        options.worker_type = (
-            WORKER_TYPE_DRIVER if is_driver else WORKER_TYPE_WORKER)
+        if worker_type in (ray.LOCAL_MODE, ray.SCRIPT_MODE):
+            self.is_driver = True
+            options.worker_type = WORKER_TYPE_DRIVER
+        elif worker_type == ray.WORKER_MODE:
+            self.is_driver = False
+            options.worker_type = WORKER_TYPE_WORKER
+        elif worker_type == ray.IO_WORKER_MODE:
+            self.is_driver = False
+            options.worker_type = WORKER_TYPE_IO_WORKER
+        else:
+            raise ValueError(f"Unknown worker type: {worker_type}")
         options.language = LANGUAGE_PYTHON
         options.store_socket = store_socket.encode("ascii")
         options.raylet_socket = raylet_socket.encode("ascii")
@@ -678,6 +731,8 @@ cdef class CoreWorker:
         options.task_execution_callback = task_execution_handler
         options.check_signals = check_signals
         options.gc_collect = gc_collect
+        options.spill_objects = spill_objects_handler
+        options.restore_spilled_objects = restore_spilled_objects_handler
         options.get_lang_stack = get_py_stack
         options.ref_counting_enabled = True
         options.is_local_mode = local_mode
@@ -725,15 +780,15 @@ cdef class CoreWorker:
         return self.plasma_event_handler
 
     def get_objects(self, object_refs, TaskID current_task_id,
-                    int64_t timeout_ms=-1):
+                    int64_t timeout_ms=-1, plasma_objects_only=False):
         cdef:
             c_vector[shared_ptr[CRayObject]] results
             CTaskID c_task_id = current_task_id.native()
             c_vector[CObjectID] c_object_ids = ObjectRefsToVector(object_refs)
-
+            c_bool _plasma_objects_only = plasma_objects_only
         with nogil:
             check_status(CCoreWorkerProcess.GetCoreWorker().Get(
-                c_object_ids, timeout_ms, &results))
+                c_object_ids, timeout_ms, &results, _plasma_objects_only))
 
         return RayObjectsToDataMetadataPairs(results)
 
@@ -771,6 +826,48 @@ cdef class CoreWorker:
         # and deal with it here.
         return data.get() == NULL
 
+    def put_file_like_object(
+            self, metadata, data_size, file_like, ObjectRef object_ref=None):
+        """Directly create a new Plasma Store object from a file like
+        object. This avoids extra memory copy.
+
+        Args:
+            metadata (bytes): The metadata of the object.
+            data_size (int): The size of the data buffer.
+            file_like: A python file object that provides the `readinto`
+                interface.
+            object_ref: The new ObjectRef.
+        """
+        cdef:
+            CObjectID c_object_id
+            shared_ptr[CBuffer] data_buf
+            shared_ptr[CBuffer] metadata_buf
+            int64_t put_threshold
+            c_bool put_small_object_in_memory_store
+            c_vector[CObjectID] c_object_id_vector
+        # TODO(suquark): This method does not support put objects to
+        # in memory store currently.
+        metadata_buf = string_to_buffer(metadata)
+        object_already_exists = self._create_put_buffer(
+            metadata_buf, data_size, object_ref,
+            ObjectRefsToVector([]),
+            &c_object_id, &data_buf)
+        if object_already_exists:
+            logger.debug("Object already exists in 'put_file_like_object'.")
+            return
+        data = Buffer.make(data_buf)
+        view = memoryview(data)
+        index = 0
+        while index < data_size:
+            bytes_read = file_like.readinto(view[index:])
+            index += bytes_read
+        with nogil:
+            # Using custom object refs is not supported because we
+            # can't track their lifecycle, so we don't pin the object
+            # in this case.
+            check_status(CCoreWorkerProcess.GetCoreWorker().Seal(
+                         c_object_id, pin_object=object_ref is None))
+
     def put_serialized_object(self, serialized_object,
                               ObjectRef object_ref=None,
                               c_bool pin_object=True):
@@ -1342,6 +1439,20 @@ cdef class CoreWorker:
             resource_name.encode("ascii"), capacity,
             CClientID.FromBinary(client_id.binary()))
 
+    def force_spill_objects(self, object_refs):
+        cdef c_vector[CObjectID] object_ids
+        object_ids = ObjectRefsToVector(object_refs)
+        with nogil:
+            check_status(CCoreWorkerProcess.GetCoreWorker()
+                         .ForceSpillObjects(object_ids))
+
+    def force_restore_spilled_objects(self, object_refs):
+        cdef c_vector[CObjectID] object_ids
+        object_ids = ObjectRefsToVector(object_refs)
+        with nogil:
+            check_status(CCoreWorkerProcess.GetCoreWorker()
+                         .ForceRestoreSpilledObjects(object_ids))
+
 cdef void async_set_result(shared_ptr[CRayObject] obj,
                            CObjectID object_ref,
                            void *future) with gil:
diff --git a/python/ray/experimental/__init__.py b/python/ray/experimental/__init__.py
index 64db9e7fc..cd0878256 100644
--- a/python/ray/experimental/__init__.py
+++ b/python/ray/experimental/__init__.py
@@ -1,10 +1,13 @@
 from .api import get, wait
 from .dynamic_resources import set_resource
+from .object_spilling import force_spill_objects, force_restore_spilled_objects
 from .placement_group import (
     placement_group, )
 __all__ = [
     "get",
     "wait",
     "set_resource",
+    "force_spill_objects",
+    "force_restore_spilled_objects",
     "placement_group",
 ]
diff --git a/python/ray/experimental/object_spilling.py b/python/ray/experimental/object_spilling.py
new file mode 100644
index 000000000..6a61ae22b
--- /dev/null
+++ b/python/ray/experimental/object_spilling.py
@@ -0,0 +1,35 @@
+import ray
+
+
+def force_spill_objects(object_refs):
+    """Force spilling objects to external storage.
+
+    Args:
+        object_refs: Object refs of the objects to be
+            spilled.
+    """
+    core_worker = ray.worker.global_worker.core_worker
+    # Make sure that the values are object refs.
+    for object_ref in object_refs:
+        if not isinstance(object_ref, ray.ObjectRef):
+            raise TypeError(
+                f"Attempting to call `force_spill_objects` on the "
+                f"value {object_ref}, which is not an ray.ObjectRef.")
+    return core_worker.force_spill_objects(object_refs)
+
+
+def force_restore_spilled_objects(object_refs):
+    """Force restoring objects from external storage.
+
+    Args:
+        object_refs: Object refs of the objects to be
+            restored.
+    """
+    core_worker = ray.worker.global_worker.core_worker
+    # Make sure that the values are object refs.
+    for object_ref in object_refs:
+        if not isinstance(object_ref, ray.ObjectRef):
+            raise TypeError(
+                f"Attempting to call `force_restore_spilled_objects` on the "
+                f"value {object_ref}, which is not an ray.ObjectRef.")
+    return core_worker.force_restore_spilled_objects(object_refs)
diff --git a/python/ray/external_storage.py b/python/ray/external_storage.py
new file mode 100644
index 000000000..093ab2bd4
--- /dev/null
+++ b/python/ray/external_storage.py
@@ -0,0 +1,129 @@
+import abc
+import os
+from typing import List
+import ray
+
+
+class ExternalStorage(metaclass=abc.ABCMeta):
+    """The base class for external storage.
+
+    This class provides some useful functions for zero-copy object
+    put/get from plasma store. Also it specifies the interface for
+    object spilling.
+    """
+
+    def _get_objects_from_store(self, object_refs):
+        worker = ray.worker.global_worker
+        ray_object_pairs = worker.core_worker.get_objects(
+            object_refs,
+            worker.current_task_id,
+            timeout_ms=0,
+            plasma_objects_only=True)
+        return ray_object_pairs
+
+    def _put_object_to_store(self, metadata, data_size, file_like, object_ref):
+        worker = ray.worker.global_worker
+        worker.core_worker.put_file_like_object(metadata, data_size, file_like,
+                                                object_ref)
+
+    @abc.abstractmethod
+    def spill_objects(self, object_refs):
+        """Spill objects to the external storage. Objects are specified
+        by their object refs.
+
+        Args:
+            object_refs: The list of the refs of the objects to be spilled.
+        Returns:
+            A list of keys corresponding to the input object refs.
+        """
+
+    @abc.abstractmethod
+    def restore_spilled_objects(self, keys: List[bytes]):
+        """Spill objects to the external storage. Objects are specified
+        by their object refs.
+
+        Args:
+            keys: A list of bytes corresponding to the spilled objects.
+        """
+
+
+class NullStorage(ExternalStorage):
+    """The class that represents an uninitialized external storage."""
+
+    def spill_objects(self, object_refs):
+        raise NotImplementedError("External storage is not initialized")
+
+    def restore_spilled_objects(self, keys):
+        raise NotImplementedError("External storage is not initialized")
+
+
+class FileSystemStorage(ExternalStorage):
+    """The class for filesystem-like external storage."""
+
+    def __init__(self, directory_path):
+        self.directory_path = directory_path
+        self.prefix = "ray_spilled_object_"
+
+    def spill_objects(self, object_refs):
+        keys = []
+        ray_object_pairs = self._get_objects_from_store(object_refs)
+        for ref, (buf, metadata) in zip(object_refs, ray_object_pairs):
+            filename = self.prefix + ref.hex()
+            with open(os.path.join(self.directory_path, filename), "wb") as f:
+                metadata_len = len(metadata)
+                buf_len = len(buf)
+                f.write(metadata_len.to_bytes(8, byteorder="little"))
+                f.write(buf_len.to_bytes(8, byteorder="little"))
+                f.write(metadata)
+                f.write(memoryview(buf))
+            keys.append(filename.encode())
+        return keys
+
+    def restore_spilled_objects(self, keys):
+        for k in keys:
+            filename = k.decode()
+            ref = ray.ObjectRef(bytes.fromhex(filename[len(self.prefix):]))
+            with open(os.path.join(self.directory_path, filename), "rb") as f:
+                metadata_len = int.from_bytes(f.read(8), byteorder="little")
+                buf_len = int.from_bytes(f.read(8), byteorder="little")
+                metadata = f.read(metadata_len)
+                # read remaining data to our buffer
+                self._put_object_to_store(metadata, buf_len, f, ref)
+
+
+_external_storage = NullStorage()
+
+
+def setup_external_storage(config):
+    """Setup the external storage according to the config."""
+    global _external_storage
+    if config:
+        storage_type = config["type"]
+        if storage_type == "filesystem":
+            _external_storage = FileSystemStorage(**config["params"])
+        else:
+            raise ValueError(f"Unknown external storage type: {storage_type}")
+    else:
+        _external_storage = NullStorage()
+
+
+def spill_objects(object_refs):
+    """Spill objects to the external storage. Objects are specified
+    by their object refs.
+
+    Args:
+        object_refs: The list of the refs of the objects to be spilled.
+    Returns:
+        A list of keys corresponding to the input object refs.
+    """
+    return _external_storage.spill_objects(object_refs)
+
+
+def restore_spilled_objects(keys: List[bytes]):
+    """Spill objects to the external storage. Objects are specified
+    by their object refs.
+
+    Args:
+        keys: A list of bytes corresponding to the spilled objects.
+    """
+    _external_storage.restore_spilled_objects(keys)
diff --git a/python/ray/includes/common.pxd b/python/ray/includes/common.pxd
index 0fdf3036f..5c8917fc9 100644
--- a/python/ray/includes/common.pxd
+++ b/python/ray/includes/common.pxd
@@ -162,6 +162,7 @@ cdef extern from "src/ray/protobuf/common.pb.h" nogil:
 cdef extern from "src/ray/protobuf/common.pb.h" nogil:
     cdef CWorkerType WORKER_TYPE_WORKER "ray::WorkerType::WORKER"
     cdef CWorkerType WORKER_TYPE_DRIVER "ray::WorkerType::DRIVER"
+    cdef CWorkerType WORKER_TYPE_IO_WORKER "ray::WorkerType::IO_WORKER"
 
 cdef extern from "src/ray/protobuf/common.pb.h" nogil:
     cdef CTaskType TASK_TYPE_NORMAL_TASK "ray::TaskType::NORMAL_TASK"
diff --git a/python/ray/includes/libcoreworker.pxd b/python/ray/includes/libcoreworker.pxd
index 120b8a40f..871e5d0c3 100644
--- a/python/ray/includes/libcoreworker.pxd
+++ b/python/ray/includes/libcoreworker.pxd
@@ -165,7 +165,8 @@ cdef extern from "ray/core_worker/core_worker.h" nogil:
                           shared_ptr[CBuffer] *data)
         CRayStatus Seal(const CObjectID &object_id, c_bool pin_object)
         CRayStatus Get(const c_vector[CObjectID] &ids, int64_t timeout_ms,
-                       c_vector[shared_ptr[CRayObject]] *results)
+                       c_vector[shared_ptr[CRayObject]] *results,
+                       c_bool plasma_objects_only)
         CRayStatus Contains(const CObjectID &object_id, c_bool *has_object)
         CRayStatus Wait(const c_vector[CObjectID] &object_ids, int num_objects,
                         int64_t timeout_ms, c_vector[c_bool] *results)
@@ -192,6 +193,9 @@ cdef extern from "ray/core_worker/core_worker.h" nogil:
         CRayStatus SetResource(const c_string &resource_name,
                                const double capacity,
                                const CClientID &client_Id)
+        CRayStatus ForceSpillObjects(const c_vector[CObjectID] &object_ids)
+        CRayStatus ForceRestoreSpilledObjects(
+                const c_vector[CObjectID] &object_ids)
 
     cdef cppclass CCoreWorkerOptions "ray::CoreWorkerOptions":
         CWorkerType worker_type
@@ -220,6 +224,8 @@ cdef extern from "ray/core_worker/core_worker.h" nogil:
          ) task_execution_callback
         (CRayStatus() nogil) check_signals
         (void() nogil) gc_collect
+        (c_vector[c_string](const c_vector[CObjectID]&) nogil) spill_objects
+        (void(const c_vector[c_string]&) nogil) restore_spilled_objects
         (void(c_string *stack_out) nogil) get_lang_stack
         c_bool ref_counting_enabled
         c_bool is_local_mode
diff --git a/python/ray/node.py b/python/ray/node.py
index b56efe498..aa4ab55d4 100644
--- a/python/ray/node.py
+++ b/python/ray/node.py
@@ -718,7 +718,8 @@ class Node:
             socket_to_use=self.socket,
             head_node=self.head,
             start_initial_python_workers_for_first_job=self._ray_params.
-            start_initial_python_workers_for_first_job)
+            start_initial_python_workers_for_first_job,
+            object_spilling_config=self._ray_params.object_spilling_config)
         assert ray_constants.PROCESS_TYPE_RAYLET not in self.all_processes
         self.all_processes[ray_constants.PROCESS_TYPE_RAYLET] = [process_info]
 
diff --git a/python/ray/parameter.py b/python/ray/parameter.py
index 07e36db26..b7eebdc26 100644
--- a/python/ray/parameter.py
+++ b/python/ray/parameter.py
@@ -145,7 +145,8 @@ class RayParams:
                  enable_object_reconstruction=False,
                  metrics_agent_port=None,
                  metrics_export_port=None,
-                 lru_evict=False):
+                 lru_evict=False,
+                 object_spilling_config=None):
         self.object_ref_seed = object_ref_seed
         self.redis_address = redis_address
         self.num_cpus = num_cpus
@@ -190,6 +191,7 @@ class RayParams:
         self._internal_config = _internal_config
         self._lru_evict = lru_evict
         self._enable_object_reconstruction = enable_object_reconstruction
+        self.object_spilling_config = object_spilling_config
         self._check_usage()
 
         # Set the internal config options for LRU eviction.
diff --git a/python/ray/services.py b/python/ray/services.py
index f5cc4d426..715cc3093 100644
--- a/python/ray/services.py
+++ b/python/ray/services.py
@@ -1289,7 +1289,8 @@ def start_raylet(redis_address,
                  fate_share=None,
                  socket_to_use=None,
                  head_node=False,
-                 start_initial_python_workers_for_first_job=False):
+                 start_initial_python_workers_for_first_job=False,
+                 object_spilling_config=None):
     """Start a raylet, which is a combined local scheduler and object manager.
 
     Args:
@@ -1398,6 +1399,10 @@ def start_raylet(redis_address,
     if load_code_from_local:
         start_worker_command += ["--load-code-from-local"]
 
+    if object_spilling_config:
+        start_worker_command.append(
+            f"--object-spilling-config={json.dumps(object_spilling_config)}")
+
     command = [
         RAYLET_EXECUTABLE,
         "--raylet_socket_name={}".format(raylet_name),
diff --git a/python/ray/tests/BUILD b/python/ray/tests/BUILD
index 50f7be593..b1572019b 100644
--- a/python/ray/tests/BUILD
+++ b/python/ray/tests/BUILD
@@ -48,6 +48,7 @@ py_test_module_list(
     "test_stress_sharded.py",
     "test_unreconstructable_errors.py",
     "test_tensorflow.py",
+    "test_object_spilling.py",
   ],
   size = "medium",
   extra_srcs = SRCS,
diff --git a/python/ray/tests/test_object_spilling.py b/python/ray/tests/test_object_spilling.py
new file mode 100644
index 000000000..cce0320ef
--- /dev/null
+++ b/python/ray/tests/test_object_spilling.py
@@ -0,0 +1,140 @@
+import json
+import random
+import time
+
+import numpy as np
+import pytest
+import ray
+
+
+def test_spill_objects_manually(shutdown_only):
+    # Limit our object store to 75 MiB of memory.
+    ray.init(
+        object_store_memory=75 * 1024 * 1024,
+        object_spilling_config={
+            "type": "filesystem",
+            "params": {
+                "directory_path": "/tmp"
+            }
+        },
+        _internal_config=json.dumps({
+            "object_store_full_max_retries": 0,
+            "max_io_workers": 4,
+        }))
+    arr = np.random.rand(1024 * 1024)  # 8 MB data
+    replay_buffer = []
+    pinned_objects = set()
+    spilled_objects = set()
+
+    # Create objects of more than 200 MiB.
+    for _ in range(25):
+        ref = None
+        while ref is None:
+            try:
+                ref = ray.put(arr)
+                replay_buffer.append(ref)
+                pinned_objects.add(ref)
+            except ray.exceptions.ObjectStoreFullError:
+                ref_to_spill = pinned_objects.pop()
+                ray.experimental.force_spill_objects([ref_to_spill])
+                spilled_objects.add(ref_to_spill)
+
+    # Spill 2 more objects so we will always have enough space for
+    # restoring objects back.
+    refs_to_spill = (pinned_objects.pop(), pinned_objects.pop())
+    ray.experimental.force_spill_objects(refs_to_spill)
+    spilled_objects.update(refs_to_spill)
+
+    # randomly sample objects
+    for _ in range(100):
+        ref = random.choice(replay_buffer)
+        if ref in spilled_objects:
+            ray.experimental.force_restore_spilled_objects([ref])
+        sample = ray.get(ref)
+        assert np.array_equal(sample, arr)
+
+
+def test_spill_objects_manually_from_workers(shutdown_only):
+    # Limit our object store to 100 MiB of memory.
+    ray.init(
+        object_store_memory=100 * 1024 * 1024,
+        object_spilling_config={
+            "type": "filesystem",
+            "params": {
+                "directory_path": "/tmp"
+            }
+        },
+        _internal_config=json.dumps({
+            "object_store_full_max_retries": 0,
+            "max_io_workers": 4,
+        }))
+
+    @ray.remote
+    def _worker():
+        arr = np.random.rand(100 * 1024)
+        ref = ray.put(arr)
+        ray.experimental.force_spill_objects([ref])
+        ray.experimental.force_restore_spilled_objects([ref])
+        assert np.array_equal(ray.get(ref), arr)
+
+    ray.get([_worker.remote() for _ in range(50)])
+
+
+def test_spill_objects_manually_with_workers(shutdown_only):
+    # Limit our object store to 75 MiB of memory.
+    ray.init(
+        object_store_memory=100 * 1024 * 1024,
+        object_spilling_config={
+            "type": "filesystem",
+            "params": {
+                "directory_path": "/tmp"
+            }
+        },
+        _internal_config=json.dumps({
+            "object_store_full_max_retries": 0,
+            "max_io_workers": 4,
+        }))
+    arrays = [np.random.rand(100 * 1024) for _ in range(50)]
+    objects = [ray.put(arr) for arr in arrays]
+
+    @ray.remote
+    def _worker(object_refs):
+        ray.experimental.force_spill_objects(object_refs)
+
+    ray.get([_worker.remote([o]) for o in objects])
+
+    for restored, arr in zip(ray.get(objects), arrays):
+        assert np.array_equal(restored, arr)
+
+
+@pytest.mark.skip(reason="have not been fully implemented")
+def test_spill_objects_automatically(shutdown_only):
+    # Limit our object store to 75 MiB of memory.
+    ray.init(
+        object_store_memory=75 * 1024 * 1024,
+        _internal_config=json.dumps({
+            "max_io_workers": 4,
+            "object_store_full_max_retries": 2,
+            "object_store_full_initial_delay_ms": 10,
+            "auto_object_spilling": True,
+        }))
+    arr = np.random.rand(1024 * 1024)  # 8 MB data
+    replay_buffer = []
+
+    # Wait raylet for starting an IO worker.
+    time.sleep(1)
+
+    # Create objects of more than 800 MiB.
+    for _ in range(100):
+        ref = None
+        while ref is None:
+            ref = ray.put(arr)
+            replay_buffer.append(ref)
+
+    print("-----------------------------------")
+
+    # randomly sample objects
+    for _ in range(1000):
+        ref = random.choice(replay_buffer)
+        sample = ray.get(ref, timeout=0)
+        assert np.array_equal(sample, arr)
diff --git a/python/ray/worker.py b/python/ray/worker.py
index ee0e073f2..f3fa074e8 100644
--- a/python/ray/worker.py
+++ b/python/ray/worker.py
@@ -53,6 +53,7 @@ from ray.utils import (_random_string, check_oversized_pickle, is_cython,
 SCRIPT_MODE = 0
 WORKER_MODE = 1
 LOCAL_MODE = 2
+IO_WORKER_MODE = 3
 
 ERROR_KEY_PREFIX = b"Error:"
 
@@ -513,7 +514,8 @@ def init(address=None,
          _internal_config=None,
          lru_evict=False,
          enable_object_reconstruction=False,
-         _metrics_export_port=None):
+         _metrics_export_port=None,
+         object_spilling_config=None):
     """
     Connect to an existing Ray cluster or start one and connect to it.
 
@@ -642,6 +644,8 @@ def init(address=None,
         _metrics_export_port(int): Port number Ray exposes system metrics
             through a Prometheus endpoint. It is currently under active
             development, and the API is subject to change.
+        object_spilling_config (str): The configuration json string for object
+            spilling I/O worker.
 
     Returns:
         Address information about the started processes.
@@ -737,7 +741,8 @@ def init(address=None,
             _internal_config=_internal_config,
             lru_evict=lru_evict,
             enable_object_reconstruction=enable_object_reconstruction,
-            metrics_export_port=_metrics_export_port)
+            metrics_export_port=_metrics_export_port,
+            object_spilling_config=object_spilling_config)
         # Start the Ray processes. We set shutdown_at_exit=False because we
         # shutdown the node in the ray.shutdown call that happens in the atexit
         # handler. We still spawn a reaper process in case the atexit handler
@@ -1206,7 +1211,7 @@ def connect(node,
     worker.redis_client = node.create_redis_client()
 
     # Initialize some fields.
-    if mode is WORKER_MODE:
+    if mode in (WORKER_MODE, IO_WORKER_MODE):
         # We should not specify the job_id if it's `WORKER_MODE`.
         assert job_id is None
         job_id = JobID.nil()
@@ -1260,7 +1265,7 @@ def connect(node,
         import __main__ as main
         driver_name = (main.__file__
                        if hasattr(main, "__file__") else "INTERACTIVE MODE")
-    elif mode == WORKER_MODE:
+    elif mode == WORKER_MODE or mode == IO_WORKER_MODE:
         # Check the RedirectOutput key in Redis and based on its value redirect
         # worker output and error to their own files.
         # This key is set in services.py when Redis is started.
@@ -1295,8 +1300,7 @@ def connect(node,
         job_config = ray.job_config.JobConfig()
     serialized_job_config = job_config.serialize()
     worker.core_worker = ray._raylet.CoreWorker(
-        (mode == SCRIPT_MODE or mode == LOCAL_MODE),
-        node.plasma_store_socket_name, node.raylet_socket_name, job_id,
+        mode, node.plasma_store_socket_name, node.raylet_socket_name, job_id,
         gcs_options, node.get_logs_dir_path(), node.node_ip_address,
         node.node_manager_port, node.raylet_ip_address, (mode == LOCAL_MODE),
         driver_name, log_stdout_file_path, log_stderr_file_path,
diff --git a/python/ray/workers/default_worker.py b/python/ray/workers/default_worker.py
index c489d48a5..6eb8ef3b6 100644
--- a/python/ray/workers/default_worker.py
+++ b/python/ray/workers/default_worker.py
@@ -1,5 +1,6 @@
 import argparse
 import json
+import time
 
 import ray
 import ray.actor
@@ -80,17 +81,47 @@ parser.add_argument(
     default=False,
     action="store_true",
     help="True if cloudpickle should be used for serialization.")
+parser.add_argument(
+    "--worker-type",
+    required=False,
+    type=str,
+    default="WORKER",
+    help="Specify the type of the worker process")
 parser.add_argument(
     "--metrics-agent-port",
     required=True,
     type=int,
     help="the port of the node's metric agent.")
+parser.add_argument(
+    "--object-spilling-config",
+    required=False,
+    type=str,
+    default="",
+    help="The configuration of object spilling. Only used by I/O workers.")
 
 if __name__ == "__main__":
     args = parser.parse_args()
 
     ray.utils.setup_logger(args.logging_level, args.logging_format)
 
+    if args.worker_type == "WORKER":
+        mode = ray.WORKER_MODE
+    elif args.worker_type == "IO_WORKER":
+        mode = ray.IO_WORKER_MODE
+    else:
+        raise ValueError("Unknown worker type: " + args.worker_type)
+
+    # NOTE(suquark): We must initialize the external storage before we
+    # connect to raylet. Otherwise we may receive requests before the
+    # external storage is intialized.
+    if mode == ray.IO_WORKER_MODE:
+        from ray import external_storage
+        if args.object_spilling_config:
+            object_spilling_config = json.loads(args.object_spilling_config)
+        else:
+            object_spilling_config = {}
+        external_storage.setup_external_storage(object_spilling_config)
+
     internal_config = {}
     if args.config_list is not None:
         config_list = args.config_list.split(",")
@@ -125,5 +156,14 @@ if __name__ == "__main__":
         spawn_reaper=False,
         connect_only=True)
     ray.worker._global_node = node
-    ray.worker.connect(node, mode=ray.WORKER_MODE)
-    ray.worker.global_worker.main_loop()
+
+    ray.worker.connect(node, mode=mode)
+    if mode == ray.WORKER_MODE:
+        ray.worker.global_worker.main_loop()
+    elif mode == ray.IO_WORKER_MODE:
+        # It is handled by another thread in the C++ core worker.
+        # We just need to keep the worker alive.
+        while True:
+            time.sleep(100000)
+    else:
+        raise ValueError(f"Unexcepted worker mode: {mode}")
diff --git a/src/ray/common/ray_config_def.h b/src/ray/common/ray_config_def.h
index 928fb98a4..6b3629710 100644
--- a/src/ray/common/ray_config_def.h
+++ b/src/ray/common/ray_config_def.h
@@ -343,3 +343,6 @@ RAY_CONFIG(bool, ownership_based_object_directory_enabled, false)
 
 // The interval where metrics are exported in milliseconds.
 RAY_CONFIG(uint64_t, metrics_report_interval_ms, 10000)
+
+/// The maximum number of I/O worker that raylet starts.
+RAY_CONFIG(int, max_io_workers, 1)
diff --git a/src/ray/core_worker/common.cc b/src/ray/core_worker/common.cc
index eb30dd04a..7470807d2 100644
--- a/src/ray/core_worker/common.cc
+++ b/src/ray/core_worker/common.cc
@@ -17,10 +17,13 @@
 namespace ray {
 
 std::string WorkerTypeString(WorkerType type) {
+  // TODO(suquark): Use proto3 utils to get the string.
   if (type == WorkerType::DRIVER) {
     return "driver";
   } else if (type == WorkerType::WORKER) {
     return "worker";
+  } else if (type == WorkerType::IO_WORKER) {
+    return "io_worker";
   }
   RAY_CHECK(false);
   return "";
diff --git a/src/ray/core_worker/core_worker.cc b/src/ray/core_worker/core_worker.cc
index edd98cda3..6300cbda9 100644
--- a/src/ray/core_worker/core_worker.cc
+++ b/src/ray/core_worker/core_worker.cc
@@ -305,9 +305,8 @@ CoreWorker::CoreWorker(const CoreWorkerOptions &options, const WorkerID &worker_
   std::unordered_map<std::string, std::string> internal_config;
   local_raylet_client_ = std::shared_ptr<raylet::RayletClient>(new raylet::RayletClient(
       io_service_, std::move(grpc_client), options_.raylet_socket, GetWorkerID(),
-      (options_.worker_type == ray::WorkerType::WORKER),
-      worker_context_.GetCurrentJobID(), options_.language, options_.node_ip_address,
-      &local_raylet_id, &assigned_port, &internal_config,
+      options_.worker_type, worker_context_.GetCurrentJobID(), options_.language,
+      options_.node_ip_address, &local_raylet_id, &assigned_port, &internal_config,
       options_.serialized_job_config));
   connected_ = true;
 
@@ -326,6 +325,7 @@ CoreWorker::CoreWorker(const CoreWorkerOptions &options, const WorkerID &worker_
   core_worker_server_->Run();
 
   // Tell the raylet the port that we are listening on.
+  // NOTE: This also marks the worker as available in Raylet.
   RAY_CHECK_OK(local_raylet_client_->AnnounceWorkerPort(core_worker_server_->GetPort()));
 
   // Set our own address.
@@ -378,8 +378,8 @@ CoreWorker::CoreWorker(const CoreWorkerOptions &options, const WorkerID &worker_
       options_.store_socket, local_raylet_client_, reference_counter_,
       options_.check_signals,
       /*evict_if_full=*/RayConfig::instance().object_pinning_enabled(),
-      boost::bind(&CoreWorker::TriggerGlobalGC, this),
-      boost::bind(&CoreWorker::CurrentCallSite, this)));
+      /*on_store_full=*/boost::bind(&CoreWorker::TriggerGlobalGC, this),
+      /*get_current_call_site=*/boost::bind(&CoreWorker::CurrentCallSite, this)));
   memory_store_.reset(new CoreWorkerMemoryStore(
       [this](const RayObject &object, const ObjectID &object_id) {
         PutObjectIntoPlasma(object, object_id);
@@ -914,7 +914,8 @@ Status CoreWorker::Seal(const ObjectID &object_id, bool pin_object,
 }
 
 Status CoreWorker::Get(const std::vector<ObjectID> &ids, const int64_t timeout_ms,
-                       std::vector<std::shared_ptr<RayObject>> *results) {
+                       std::vector<std::shared_ptr<RayObject>> *results,
+                       bool plasma_objects_only) {
   results->resize(ids.size(), nullptr);
 
   absl::flat_hash_set<ObjectID> plasma_object_ids;
@@ -924,20 +925,24 @@ Status CoreWorker::Get(const std::vector<ObjectID> &ids, const int64_t timeout_m
   absl::flat_hash_map<ObjectID, std::shared_ptr<RayObject>> result_map;
   auto start_time = current_time_ms();
 
-  if (!memory_object_ids.empty()) {
-    RAY_RETURN_NOT_OK(memory_store_->Get(memory_object_ids, timeout_ms, worker_context_,
-                                         &result_map, &got_exception));
-  }
-
-  // Erase any objects that were promoted to plasma from the results. These get
-  // requests will be retried at the plasma store.
-  for (auto it = result_map.begin(); it != result_map.end();) {
-    auto current = it++;
-    if (current->second->IsInPlasmaError()) {
-      RAY_LOG(DEBUG) << current->first << " in plasma, doing fetch-and-get";
-      plasma_object_ids.insert(current->first);
-      result_map.erase(current);
+  if (!plasma_objects_only) {
+    if (!memory_object_ids.empty()) {
+      RAY_RETURN_NOT_OK(memory_store_->Get(memory_object_ids, timeout_ms, worker_context_,
+                                           &result_map, &got_exception));
     }
+
+    // Erase any objects that were promoted to plasma from the results. These get
+    // requests will be retried at the plasma store.
+    for (auto it = result_map.begin(); it != result_map.end();) {
+      auto current = it++;
+      if (current->second->IsInPlasmaError()) {
+        RAY_LOG(DEBUG) << current->first << " in plasma, doing fetch-and-get";
+        plasma_object_ids.insert(current->first);
+        result_map.erase(current);
+      }
+    }
+  } else {
+    plasma_object_ids = std::move(memory_object_ids);
   }
 
   if (!got_exception) {
@@ -1169,6 +1174,14 @@ Status CoreWorker::SetResource(const std::string &resource_name, const double ca
   return local_raylet_client_->SetResource(resource_name, capacity, client_id);
 }
 
+Status CoreWorker::ForceSpillObjects(const std::vector<ObjectID> &object_ids) {
+  return local_raylet_client_->ForceSpillObjects(object_ids);
+}
+
+Status CoreWorker::ForceRestoreSpilledObjects(const std::vector<ObjectID> &object_ids) {
+  return local_raylet_client_->ForceRestoreSpilledObjects(object_ids);
+}
+
 std::unordered_map<std::string, double> AddPlacementGroupConstraint(
     const std::unordered_map<std::string, double> &resources,
     PlacementGroupID placement_group_id, int64_t bundle_index) {
@@ -2103,6 +2116,44 @@ void CoreWorker::HandleLocalGC(const rpc::LocalGCRequest &request,
   }
 }
 
+void CoreWorker::HandleSpillObjects(const rpc::SpillObjectsRequest &request,
+                                    rpc::SpillObjectsReply *reply,
+                                    rpc::SendReplyCallback send_reply_callback) {
+  if (options_.spill_objects != nullptr) {
+    std::vector<ObjectID> object_ids_to_spill;
+    object_ids_to_spill.reserve(request.object_ids_to_spill_size());
+    for (const auto &id_binary : request.object_ids_to_spill()) {
+      object_ids_to_spill.push_back(ObjectID::FromBinary(id_binary));
+    }
+    std::vector<std::string> object_urls = options_.spill_objects(object_ids_to_spill);
+    for (size_t i = 0; i < object_urls.size(); i++) {
+      reply->add_spilled_objects_url(std::move(object_urls[i]));
+    }
+    send_reply_callback(Status::OK(), nullptr, nullptr);
+  } else {
+    send_reply_callback(Status::NotImplemented("Spill objects callback not defined"),
+                        nullptr, nullptr);
+  }
+}
+
+void CoreWorker::HandleRestoreSpilledObjects(
+    const rpc::RestoreSpilledObjectsRequest &request,
+    rpc::RestoreSpilledObjectsReply *reply, rpc::SendReplyCallback send_reply_callback) {
+  if (options_.restore_spilled_objects != nullptr) {
+    std::vector<std::string> spilled_objects_url;
+    spilled_objects_url.reserve(request.spilled_objects_url_size());
+    for (const auto &url : request.spilled_objects_url()) {
+      spilled_objects_url.push_back(url);
+    }
+    options_.restore_spilled_objects(spilled_objects_url);
+    send_reply_callback(Status::OK(), nullptr, nullptr);
+  } else {
+    send_reply_callback(
+        Status::NotImplemented("Restore spilled objects callback not defined"), nullptr,
+        nullptr);
+  }
+}
+
 void CoreWorker::YieldCurrentFiber(FiberEvent &event) {
   RAY_CHECK(worker_context_.CurrentActorIsAsync());
   boost::this_fiber::yield();
diff --git a/src/ray/core_worker/core_worker.h b/src/ray/core_worker/core_worker.h
index c1eb621bb..31211d867 100644
--- a/src/ray/core_worker/core_worker.h
+++ b/src/ray/core_worker/core_worker.h
@@ -105,6 +105,10 @@ struct CoreWorkerOptions {
   /// runtime. This is required to free distributed references that may otherwise
   /// be held up in garbage objects.
   std::function<void()> gc_collect;
+  /// Application-language callback to spill objects to external storage.
+  std::function<std::vector<std::string>(const std::vector<ObjectID> &)> spill_objects;
+  /// Application-language callback to restore objects from external storage.
+  std::function<void(const std::vector<std::string> &)> restore_spilled_objects;
   /// Language worker callback to get the current call stack.
   std::function<void(std::string *)> get_lang_stack;
   // Function that tries to interrupt the currently running Python thread.
@@ -490,9 +494,11 @@ class CoreWorker : public rpc::CoreWorkerServiceHandler {
   /// \param[in] ids IDs of the objects to get.
   /// \param[in] timeout_ms Timeout in milliseconds, wait infinitely if it's negative.
   /// \param[out] results Result list of objects data.
+  /// \param[in] plasma_objects_only Only get objects from Plasma Store.
   /// \return Status.
   Status Get(const std::vector<ObjectID> &ids, const int64_t timeout_ms,
-             std::vector<std::shared_ptr<RayObject>> *results);
+             std::vector<std::shared_ptr<RayObject>> *results,
+             bool plasma_objects_only = false);
 
   /// Return whether or not the object store contains the given object.
   ///
@@ -580,6 +586,16 @@ class CoreWorker : public rpc::CoreWorkerServiceHandler {
   Status SetResource(const std::string &resource_name, const double capacity,
                      const ClientID &client_id);
 
+  /// Force spilling objects to external storage.
+  /// \param[in] object_ids The objects to be spilled.
+  /// \return Status
+  Status ForceSpillObjects(const std::vector<ObjectID> &object_ids);
+
+  /// Restore objects from external storage.
+  /// \param[in] object_ids The objects to be restored.
+  /// \return Status
+  Status ForceRestoreSpilledObjects(const std::vector<ObjectID> &object_ids);
+
   /// Submit a normal task.
   ///
   /// \param[in] function The remote function to execute.
@@ -809,6 +825,16 @@ class CoreWorker : public rpc::CoreWorkerServiceHandler {
   void HandleLocalGC(const rpc::LocalGCRequest &request, rpc::LocalGCReply *reply,
                      rpc::SendReplyCallback send_reply_callback) override;
 
+  // Spill objects to external storage.
+  void HandleSpillObjects(const rpc::SpillObjectsRequest &request,
+                          rpc::SpillObjectsReply *reply,
+                          rpc::SendReplyCallback send_reply_callback) override;
+
+  // Restore objects from external storage.
+  void HandleRestoreSpilledObjects(const rpc::RestoreSpilledObjectsRequest &request,
+                                   rpc::RestoreSpilledObjectsReply *reply,
+                                   rpc::SendReplyCallback send_reply_callback) override;
+
   ///
   /// Public methods related to async actor call. This should only be used when
   /// the actor is (1) direct actor and (2) using asyncio mode.
diff --git a/src/ray/core_worker/lib/java/io_ray_runtime_RayNativeRuntime.cc b/src/ray/core_worker/lib/java/io_ray_runtime_RayNativeRuntime.cc
index 26a2f7dba..e1acf26f5 100644
--- a/src/ray/core_worker/lib/java/io_ray_runtime_RayNativeRuntime.cc
+++ b/src/ray/core_worker/lib/java/io_ray_runtime_RayNativeRuntime.cc
@@ -225,6 +225,8 @@ JNIEXPORT void JNICALL Java_io_ray_runtime_RayNativeRuntime_nativeInitialize(
       task_execution_callback,                       // task_execution_callback
       nullptr,                                       // check_signals
       gc_collect,                                    // gc_collect
+      nullptr,                                       // spill_objects
+      nullptr,                                       // restore_spilled_objects
       nullptr,                                       // get_lang_stack
       nullptr,                                       // kill_main
       true,                                          // ref_counting_enabled
diff --git a/src/ray/core_worker/test/core_worker_test.cc b/src/ray/core_worker/test/core_worker_test.cc
index 0c2e2a942..af87dd5ef 100644
--- a/src/ray/core_worker/test/core_worker_test.cc
+++ b/src/ray/core_worker/test/core_worker_test.cc
@@ -158,6 +158,8 @@ class CoreWorkerTest : public ::testing::Test {
           nullptr,                        // task_execution_callback
           nullptr,                        // check_signals
           nullptr,                        // gc_collect
+          nullptr,                        // spill_objects
+          nullptr,                        // restore_spilled_objects
           nullptr,                        // get_lang_stack
           nullptr,                        // kill_main
           true,                           // ref_counting_enabled
diff --git a/src/ray/core_worker/test/mock_worker.cc b/src/ray/core_worker/test/mock_worker.cc
index cf4cc26c2..d705d97e5 100644
--- a/src/ray/core_worker/test/mock_worker.cc
+++ b/src/ray/core_worker/test/mock_worker.cc
@@ -54,6 +54,8 @@ class MockWorker {
                   _7),  // task_execution_callback
         nullptr,        // check_signals
         nullptr,        // gc_collect
+        nullptr,        // spill_objects
+        nullptr,        // restore_spilled_objects
         nullptr,        // get_lang_stack
         nullptr,        // kill_main
         true,           // ref_counting_enabled
diff --git a/src/ray/protobuf/common.proto b/src/ray/protobuf/common.proto
index 8745b3a57..261fd3f44 100644
--- a/src/ray/protobuf/common.proto
+++ b/src/ray/protobuf/common.proto
@@ -29,6 +29,7 @@ enum Language {
 enum WorkerType {
   WORKER = 0;
   DRIVER = 1;
+  IO_WORKER = 2;
 }
 
 // Type of a task.
diff --git a/src/ray/protobuf/core_worker.proto b/src/ray/protobuf/core_worker.proto
index 80f7ce30e..007c97b70 100644
--- a/src/ray/protobuf/core_worker.proto
+++ b/src/ray/protobuf/core_worker.proto
@@ -294,6 +294,24 @@ message PlasmaObjectReadyRequest {
 message PlasmaObjectReadyReply {
 }
 
+message SpillObjectsRequest {
+  // The IDs of objects to be spilled.
+  repeated bytes object_ids_to_spill = 1;
+}
+
+message SpillObjectsReply {
+  // The URLs of spilled objects.
+  repeated string spilled_objects_url = 1;
+}
+
+message RestoreSpilledObjectsRequest {
+  // The URLs of spilled objects.
+  repeated string spilled_objects_url = 1;
+}
+
+message RestoreSpilledObjectsReply {
+}
+
 service CoreWorkerService {
   // Push a task directly to this worker from another.
   rpc PushTask(PushTaskRequest) returns (PushTaskReply);
@@ -332,6 +350,11 @@ service CoreWorkerService {
   rpc WaitForRefRemoved(WaitForRefRemovedRequest) returns (WaitForRefRemovedReply);
   // Trigger local GC on the worker.
   rpc LocalGC(LocalGCRequest) returns (LocalGCReply);
+  // Spill objects to external storage. Caller: raylet; callee: I/O worker.
+  rpc SpillObjects(SpillObjectsRequest) returns (SpillObjectsReply);
+  // Restore spilled objects from external storage. Caller: raylet; callee: I/O worker.
+  rpc RestoreSpilledObjects(RestoreSpilledObjectsRequest)
+      returns (RestoreSpilledObjectsReply);
   // Notification from raylet that an object ID is available in local plasma.
   rpc PlasmaObjectReady(PlasmaObjectReadyRequest) returns (PlasmaObjectReadyReply);
 }
diff --git a/src/ray/raylet/format/node_manager.fbs b/src/ray/raylet/format/node_manager.fbs
index c5457804f..34cda671c 100644
--- a/src/ray/raylet/format/node_manager.fbs
+++ b/src/ray/raylet/format/node_manager.fbs
@@ -82,6 +82,12 @@ enum MessageType:int {
   SetResourceRequest,
   // Subscribe to Plasma updates
   SubscribePlasmaReady,
+  // Manually spill objects to external storage.
+  ForceSpillObjectsRequest,
+  ForceSpillObjectsReply,
+  // Manually restore objects from external storage.
+  ForceRestoreSpilledObjectsRequest,
+  ForceRestoreSpilledObjectsReply,
 }
 
 table TaskExecutionSpecification {
@@ -126,8 +132,9 @@ table ResourceIdSetInfos {
 // This struct is used to register a new worker with the raylet.
 // It is shipped as part of raylet_connect.
 table RegisterClientRequest {
-  // True if the client is a worker and false if the client is a driver.
-  is_worker: bool;
+  // Type of the worker.
+  // TODO(suquark): Use `WorkerType` in `common.proto`.
+  worker_type: int;
   // The ID of the worker or driver.
   worker_id: string;
   // The process ID of this worker.
@@ -292,3 +299,19 @@ table SubscribePlasmaReady {
   // ObjectID to wait for
   object_id: string;
 }
+
+table ForceSpillObjectsRequest {
+  // List of object IDs to be spilled to external storage.
+  object_ids: [string];
+}
+
+table ForceSpillObjectsReply {
+}
+
+table ForceRestoreSpilledObjectsRequest {
+  // List of object IDs to be restored from external storage.
+  object_ids: [string];
+}
+
+table ForceRestoreSpilledObjectsReply {
+}
diff --git a/src/ray/raylet/node_manager.cc b/src/ray/raylet/node_manager.cc
index f5602dffa..6e9458658 100644
--- a/src/ray/raylet/node_manager.cc
+++ b/src/ray/raylet/node_manager.cc
@@ -507,6 +507,89 @@ void NodeManager::DoLocalGC() {
   }
 }
 
+void NodeManager::SpillObjects(const std::vector<ObjectID> &objects_ids_to_spill,
+                               std::function<void(const ray::Status &)> callback) {
+  std::vector<ObjectID> objects_ids;
+  for (const auto &id : objects_ids_to_spill) {
+    // Do not spill already spilled objects.
+    if (spilled_objects_.count(id) == 0) {
+      objects_ids.push_back(id);
+    }
+  }
+  if (objects_ids.empty()) {
+    if (callback) {
+      callback(Status::OK());
+    }
+    return;
+  }
+  worker_pool_.PopIOWorker(
+      [this, objects_ids, callback](std::shared_ptr<WorkerInterface> io_worker) {
+        RAY_LOG(DEBUG) << "Sending object spilling request";
+        rpc::SpillObjectsRequest request;
+        for (const auto &object_id : objects_ids) {
+          request.add_object_ids_to_spill(object_id.Binary());
+        }
+        io_worker->rpc_client()->SpillObjects(
+            request, [this, objects_ids, callback, io_worker](
+                         const ray::Status &status, const rpc::SpillObjectsReply &r) {
+              worker_pool_.PushIOWorker(io_worker);
+              if (!status.ok()) {
+                RAY_LOG(ERROR) << "Failed to send object spilling request: "
+                               << status.ToString();
+              } else {
+                RAY_CHECK(static_cast<size_t>(r.spilled_objects_url_size()) ==
+                          objects_ids.size());
+                for (size_t i = 0; i < objects_ids.size(); ++i) {
+                  const ObjectID &object_id = objects_ids[i];
+                  const std::string &object_url = r.spilled_objects_url(i);
+                  // TODO(suquark): write to object directory.
+                  spilled_objects_[object_id] = object_url;
+                  auto search = pinned_objects_.find(object_id);
+                  if (search != pinned_objects_.end()) {
+                    pinned_objects_.erase(search);
+                  } else {
+                    RAY_LOG(ERROR)
+                        << "The spilled object " << object_id.Hex() << " is not pinned.";
+                  }
+                }
+              }
+              if (callback) {
+                callback(status);
+              }
+            });
+      });
+}
+
+void NodeManager::RestoreSpilledObjects(
+    const std::vector<ObjectID> &object_ids,
+    std::function<void(const ray::Status &)> callback) {
+  std::vector<std::string> object_urls;
+  object_urls.reserve(object_ids.size());
+  for (const auto &object_id : object_ids) {
+    object_urls.push_back(spilled_objects_[object_id]);
+  }
+  worker_pool_.PopIOWorker([this, object_urls,
+                            callback](std::shared_ptr<WorkerInterface> io_worker) {
+    RAY_LOG(DEBUG) << "Sending restore spilled object request";
+    rpc::RestoreSpilledObjectsRequest request;
+    for (const auto &url : object_urls) {
+      request.add_spilled_objects_url(std::move(url));
+    }
+    io_worker->rpc_client()->RestoreSpilledObjects(
+        request, [this, callback, io_worker](const ray::Status &status,
+                                             const rpc::RestoreSpilledObjectsReply &r) {
+          worker_pool_.PushIOWorker(io_worker);
+          if (!status.ok()) {
+            RAY_LOG(ERROR) << "Failed to send restore spilled object request: "
+                           << status.ToString();
+          }
+          if (callback) {
+            callback(status);
+          }
+        });
+  });
+}
+
 // TODO(edoakes): this function is problematic because it both sends warnings spuriously
 // under normal conditions and sometimes doesn't send a warning under actual deadlock
 // conditions. The current logic is to push a warning when: all running tasks are
@@ -1144,7 +1227,41 @@ void NodeManager::ProcessClientMessage(const std::shared_ptr<ClientConnection> &
   case protocol::MessageType::SubscribePlasmaReady: {
     ProcessSubscribePlasmaReady(client, message_data);
   } break;
-
+  case protocol::MessageType::ForceSpillObjectsRequest: {
+    auto message = flatbuffers::GetRoot<protocol::ForceSpillObjectsRequest>(message_data);
+    std::vector<ObjectID> object_ids = from_flatbuf<ObjectID>(*message->object_ids());
+    SpillObjects(object_ids, [this, client](const ray::Status &status) {
+      flatbuffers::FlatBufferBuilder fbb;
+      flatbuffers::Offset<protocol::ForceSpillObjectsReply> reply =
+          protocol::CreateForceSpillObjectsReply(fbb);
+      fbb.Finish(reply);
+      auto reply_status = client->WriteMessage(
+          static_cast<int64_t>(protocol::MessageType::ForceSpillObjectsReply),
+          fbb.GetSize(), fbb.GetBufferPointer());
+      if (!reply_status.ok()) {
+        // We failed to write to the client, so disconnect the client.
+        ProcessDisconnectClientMessage(client);
+      }
+    });
+  } break;
+  case protocol::MessageType::ForceRestoreSpilledObjectsRequest: {
+    auto message =
+        flatbuffers::GetRoot<protocol::ForceRestoreSpilledObjectsRequest>(message_data);
+    std::vector<ObjectID> object_ids = from_flatbuf<ObjectID>(*message->object_ids());
+    RestoreSpilledObjects(object_ids, [this, client](const ray::Status &status) {
+      flatbuffers::FlatBufferBuilder fbb;
+      flatbuffers::Offset<protocol::ForceRestoreSpilledObjectsReply> reply =
+          protocol::CreateForceRestoreSpilledObjectsReply(fbb);
+      fbb.Finish(reply);
+      auto reply_status = client->WriteMessage(
+          static_cast<int64_t>(protocol::MessageType::ForceRestoreSpilledObjectsReply),
+          fbb.GetSize(), fbb.GetBufferPointer());
+      if (!reply_status.ok()) {
+        // We failed to write to the client, so disconnect the client.
+        ProcessDisconnectClientMessage(client);
+      }
+    });
+  } break;
   default:
     RAY_LOG(FATAL) << "Received unexpected message type " << message_type;
   }
@@ -1162,8 +1279,10 @@ void NodeManager::ProcessRegisterClientRequestMessage(
   WorkerID worker_id = from_flatbuf<WorkerID>(*message->worker_id());
   pid_t pid = message->worker_pid();
   std::string worker_ip_address = string_from_flatbuf(*message->ip_address());
+  // TODO(suquark): Use `WorkerType` in `common.proto` without type converting.
+  rpc::WorkerType worker_type = static_cast<rpc::WorkerType>(message->worker_type());
   auto worker = std::dynamic_pointer_cast<WorkerInterface>(std::make_shared<Worker>(
-      worker_id, language, worker_ip_address, client, client_call_manager_));
+      worker_id, language, worker_type, worker_ip_address, client, client_call_manager_));
 
   auto send_reply_callback = [this, client](int assigned_port) {
     flatbuffers::FlatBufferBuilder fbb;
@@ -1187,7 +1306,8 @@ void NodeManager::ProcessRegisterClientRequestMessage(
         });
   };
 
-  if (message->is_worker()) {
+  if (worker_type == rpc::WorkerType::WORKER ||
+      worker_type == rpc::WorkerType::IO_WORKER) {
     // Register the new worker.
     RAY_UNUSED(worker_pool_.RegisterWorker(worker, pid, send_reply_callback));
   } else {
@@ -1238,6 +1358,13 @@ void NodeManager::HandleWorkerAvailable(const std::shared_ptr<ClientConnection>
 
 void NodeManager::HandleWorkerAvailable(const std::shared_ptr<WorkerInterface> &worker) {
   RAY_CHECK(worker);
+
+  if (worker->GetWorkerType() == rpc::WorkerType::IO_WORKER) {
+    // Return the worker to the idle pool.
+    worker_pool_.PushIOWorker(worker);
+    return;
+  }
+
   bool worker_idle = true;
 
   // If the worker was assigned a task, mark it as finished.
@@ -1400,15 +1527,23 @@ void NodeManager::ProcessFetchOrReconstructMessage(
   const auto refs =
       FlatbufferToObjectReference(*message->object_ids(), *message->owner_addresses());
   if (message->fetch_only()) {
+    std::vector<ObjectID> spilled_object_ids;
     for (const auto &ref : refs) {
       ObjectID object_id = ObjectID::FromBinary(ref.object_id());
       // If only a fetch is required, then do not subscribe to the
       // dependencies to the task dependency manager.
       if (!task_dependency_manager_.CheckObjectLocal(object_id)) {
-        // Fetch the object if it's not already local.
-        RAY_CHECK_OK(object_manager_.Pull(object_id, ref.owner_address()));
+        if (spilled_objects_.count(object_id) > 0) {
+          spilled_object_ids.push_back(object_id);
+        } else {
+          // Fetch the object if it's not already local.
+          RAY_CHECK_OK(object_manager_.Pull(object_id, ref.owner_address()));
+        }
       }
     }
+    if (spilled_object_ids.size() > 0) {
+      RestoreSpilledObjects(spilled_object_ids);
+    }
   } else {
     // The values are needed. Add all requested objects to the list to
     // subscribe to in the task dependency manager. These objects will be
diff --git a/src/ray/raylet/node_manager.h b/src/ray/raylet/node_manager.h
index f8b8a6797..15192640e 100644
--- a/src/ray/raylet/node_manager.h
+++ b/src/ray/raylet/node_manager.h
@@ -614,6 +614,16 @@ class NodeManager : public rpc::NodeManagerServiceHandler {
   /// Trigger local GC on each worker of this raylet.
   void DoLocalGC();
 
+  /// Spill objects to external storage.
+  /// \param objects_ids_to_spill The objects to be spilled.
+  void SpillObjects(const std::vector<ObjectID> &objects_ids_to_spill,
+                    std::function<void(const ray::Status &)> callback = nullptr);
+
+  /// Restore spilled objects from external storage.
+  /// \param object_ids Objects to be restored.
+  void RestoreSpilledObjects(const std::vector<ObjectID> &object_ids,
+                             std::function<void(const ray::Status &)> callback = nullptr);
+
   /// Push an error to the driver if this node is full of actors and so we are
   /// unable to schedule new tasks or actors at all.
   void WarnResourceDeadlock();
@@ -699,7 +709,9 @@ class NodeManager : public rpc::NodeManagerServiceHandler {
   /// A mapping from actor ID to registration information about that actor
   /// (including which node manager owns it).
   std::unordered_map<ActorID, ActorRegistration> actor_registry_;
-
+  /// A mapping from ObjectIDs to external object URLs for spilled objects.
+  /// TODO(suquark): Move it into object directory.
+  absl::flat_hash_map<ObjectID, std::string> spilled_objects_;
   /// This map stores actor ID to the ID of the checkpoint that will be used to
   /// restore the actor.
   std::unordered_map<ActorID, ActorCheckpointID> checkpoint_id_to_restore_;
diff --git a/src/ray/raylet/scheduling/cluster_task_manager_test.cc b/src/ray/raylet/scheduling/cluster_task_manager_test.cc
index 6133c0dae..27dbd5b24 100644
--- a/src/ray/raylet/scheduling/cluster_task_manager_test.cc
+++ b/src/ray/raylet/scheduling/cluster_task_manager_test.cc
@@ -62,6 +62,8 @@ class MockWorker : public WorkerInterface {
 
   WorkerID WorkerId() const { return worker_id_; }
 
+  rpc::WorkerType GetWorkerType() const { return rpc::WorkerType::WORKER; }
+
   int Port() const { return port_; }
 
   void SetOwnerAddress(const rpc::Address &address) { address_ = address; }
diff --git a/src/ray/raylet/worker.cc b/src/ray/raylet/worker.cc
index c37685f1a..d062f49ff 100644
--- a/src/ray/raylet/worker.cc
+++ b/src/ray/raylet/worker.cc
@@ -27,11 +27,12 @@ namespace raylet {
 
 /// A constructor responsible for initializing the state of a worker.
 Worker::Worker(const WorkerID &worker_id, const Language &language,
-               const std::string &ip_address,
+               rpc::WorkerType worker_type, const std::string &ip_address,
                std::shared_ptr<ClientConnection> connection,
                rpc::ClientCallManager &client_call_manager)
     : worker_id_(worker_id),
       language_(language),
+      worker_type_(worker_type),
       ip_address_(ip_address),
       assigned_port_(-1),
       port_(-1),
@@ -41,6 +42,8 @@ Worker::Worker(const WorkerID &worker_id, const Language &language,
       client_call_manager_(client_call_manager),
       is_detached_actor_(false) {}
 
+rpc::WorkerType Worker::GetWorkerType() const { return worker_type_; }
+
 void Worker::MarkDead() { dead_ = true; }
 
 bool Worker::IsDead() const { return dead_; }
diff --git a/src/ray/raylet/worker.h b/src/ray/raylet/worker.h
index cb845fbaf..61d811d4f 100644
--- a/src/ray/raylet/worker.h
+++ b/src/ray/raylet/worker.h
@@ -37,6 +37,7 @@ class WorkerInterface {
  public:
   /// A destructor responsible for freeing all worker state.
   virtual ~WorkerInterface() {}
+  virtual rpc::WorkerType GetWorkerType() const = 0;
   virtual void MarkDead() = 0;
   virtual bool IsDead() const = 0;
   virtual void MarkBlocked() = 0;
@@ -117,11 +118,12 @@ class Worker : public WorkerInterface {
  public:
   /// A constructor that initializes a worker object.
   /// NOTE: You MUST manually set the worker process.
-  Worker(const WorkerID &worker_id, const Language &language,
+  Worker(const WorkerID &worker_id, const Language &language, rpc::WorkerType worker_type,
          const std::string &ip_address, std::shared_ptr<ClientConnection> connection,
          rpc::ClientCallManager &client_call_manager);
   /// A destructor responsible for freeing all worker state.
   ~Worker() {}
+  rpc::WorkerType GetWorkerType() const;
   void MarkDead();
   bool IsDead() const;
   void MarkBlocked();
@@ -215,6 +217,8 @@ class Worker : public WorkerInterface {
   Process proc_;
   /// The language type of this worker.
   Language language_;
+  /// The type of the worker.
+  rpc::WorkerType worker_type_;
   /// IP address of this worker.
   std::string ip_address_;
   /// Port assigned to this worker by the raylet. If this is 0, the actual
diff --git a/src/ray/raylet/worker_pool.cc b/src/ray/raylet/worker_pool.cc
index c8ebdc0cc..e07ffb41d 100644
--- a/src/ray/raylet/worker_pool.cc
+++ b/src/ray/raylet/worker_pool.cc
@@ -131,7 +131,7 @@ void WorkerPool::Start(int num_workers) {
     int num_worker_processes = static_cast<int>(
         std::ceil(static_cast<double>(num_workers) / state.num_workers_per_process));
     for (int i = 0; i < num_worker_processes; i++) {
-      StartWorkerProcess(entry.first, JobID::Nil());
+      StartWorkerProcess(entry.first, ray::rpc::WorkerType::WORKER, JobID::Nil());
     }
   }
 }
@@ -165,7 +165,9 @@ uint32_t WorkerPool::Size(const Language &language) const {
   }
 }
 
-Process WorkerPool::StartWorkerProcess(const Language &language, const JobID &job_id,
+Process WorkerPool::StartWorkerProcess(const Language &language,
+                                       const rpc::WorkerType worker_type,
+                                       const JobID &job_id,
                                        std::vector<std::string> dynamic_options) {
   rpc::JobConfig *job_config = nullptr;
   if (RayConfig::instance().enable_multi_tenancy()) {
@@ -186,6 +188,8 @@ Process WorkerPool::StartWorkerProcess(const Language &language, const JobID &jo
   for (auto &entry : state.starting_worker_processes) {
     starting_workers += entry.second;
   }
+
+  // Here we consider both task workers and I/O workers.
   if (starting_workers >= maximum_startup_concurrency_) {
     // Workers have been started, but not registered. Force start disabled -- returning.
     RAY_LOG(DEBUG) << "Worker not started, " << starting_workers
@@ -271,6 +275,13 @@ Process WorkerPool::StartWorkerProcess(const Language &language, const JobID &jo
     RAY_CHECK(worker_raylet_config_placeholder_found)
         << "The " << kWorkerRayletConfigPlaceholder
         << " placeholder is not found in worker command.";
+  } else if (language == Language::PYTHON) {
+    RAY_CHECK(worker_type == rpc::WorkerType::WORKER ||
+              worker_type == rpc::WorkerType::IO_WORKER);
+    if (worker_type == rpc::WorkerType::IO_WORKER) {
+      // Without "--worker-type", by default the worker type is rpc::WorkerType::WORKER.
+      worker_command_args.push_back("--worker-type=" + rpc::WorkerType_Name(worker_type));
+    }
   }
 
   std::map<std::string, std::string> env;
@@ -286,19 +297,23 @@ Process WorkerPool::StartWorkerProcess(const Language &language, const JobID &jo
   }
   RAY_LOG(DEBUG) << "Started worker process of " << workers_to_start
                  << " worker(s) with pid " << proc.GetId();
-  MonitorStartingWorkerProcess(proc, language);
+  MonitorStartingWorkerProcess(proc, language, worker_type);
   state.starting_worker_processes.emplace(proc, workers_to_start);
+  if (worker_type == rpc::WorkerType::IO_WORKER) {
+    state.num_starting_io_workers++;
+  }
   return proc;
 }
 
 void WorkerPool::MonitorStartingWorkerProcess(const Process &proc,
-                                              const Language &language) {
+                                              const Language &language,
+                                              const rpc::WorkerType worker_type) {
   auto timer = std::make_shared<boost::asio::deadline_timer>(
       *io_service_, boost::posix_time::seconds(
                         RayConfig::instance().worker_register_timeout_seconds()));
   // Capture timer in lambda to copy it once, so that it can avoid destructing timer.
   timer->async_wait(
-      [timer, language, proc, this](const boost::system::error_code e) -> void {
+      [timer, language, proc, worker_type, this](const boost::system::error_code e) {
         // check the error code.
         auto &state = this->GetStateForLanguage(language);
         // Since this process times out to start, remove it from starting_worker_processes
@@ -308,6 +323,12 @@ void WorkerPool::MonitorStartingWorkerProcess(const Process &proc,
           RAY_LOG(INFO) << "Some workers of the worker process(" << proc.GetId()
                         << ") have not registered to raylet within timeout.";
           state.starting_worker_processes.erase(it);
+          if (worker_type == rpc::WorkerType::IO_WORKER) {
+            // Mark the I/O worker as failed.
+            state.num_starting_io_workers--;
+          }
+          // We may have places to start more workers now.
+          TryStartIOWorkers(language, state);
           starting_worker_timeout_callback_();
         }
       });
@@ -397,16 +418,23 @@ Status WorkerPool::RegisterWorker(const std::shared_ptr<WorkerInterface> &worker
     status = Status::Invalid("Unknown worker");
   } else {
     RAY_RETURN_NOT_OK(GetNextFreePort(&port));
-    RAY_LOG(DEBUG) << "Registering worker with pid " << pid << ", port: " << port;
+    RAY_LOG(DEBUG) << "Registering worker with pid " << pid << ", port: " << port
+                   << ", worker_type: " << rpc::WorkerType_Name(worker->GetWorkerType());
     worker->SetAssignedPort(port);
     worker->SetProcess(it->first);
     it->second--;
     if (it->second == 0) {
       state.starting_worker_processes.erase(it);
+      // We may have slots to start more workers now.
+      TryStartIOWorkers(worker->GetLanguage(), state);
     }
 
     RAY_CHECK(worker->GetProcess().GetId() == pid);
     state.registered_workers.insert(worker);
+    if (worker->GetWorkerType() == rpc::WorkerType::IO_WORKER) {
+      state.registered_io_workers.insert(worker);
+      state.num_starting_io_workers--;
+    }
 
     if (RayConfig::instance().enable_multi_tenancy()) {
       auto dedicated_workers_it = state.worker_pids_to_assigned_jobs.find(pid);
@@ -468,7 +496,7 @@ Status WorkerPool::RegisterDriver(const std::shared_ptr<WorkerInterface> &driver
           delay_callback = true;
           // Start initial Python workers for the first job.
           for (int i = 0; i < num_initial_python_workers_for_first_job_; i++) {
-            StartWorkerProcess(Language::PYTHON, job_id);
+            StartWorkerProcess(Language::PYTHON, rpc::WorkerType::WORKER, job_id);
           }
         }
       }
@@ -509,12 +537,39 @@ std::shared_ptr<WorkerInterface> WorkerPool::GetRegisteredDriver(
   return nullptr;
 }
 
+void WorkerPool::PushIOWorker(const std::shared_ptr<WorkerInterface> &worker) {
+  auto &state = GetStateForLanguage(worker->GetLanguage());
+  RAY_CHECK(worker->GetWorkerType() == rpc::WorkerType::IO_WORKER);
+  RAY_LOG(DEBUG) << "Pushing an IO worker to the worker pool.";
+  if (state.pending_io_tasks.empty()) {
+    state.idle_io_workers.push(worker);
+  } else {
+    auto callback = state.pending_io_tasks.front();
+    state.pending_io_tasks.pop();
+    callback(worker);
+  }
+}
+
+void WorkerPool::PopIOWorker(
+    std::function<void(std::shared_ptr<WorkerInterface>)> callback) {
+  auto &state = GetStateForLanguage(Language::PYTHON);
+  if (state.idle_io_workers.empty()) {
+    // We must fill the pending task first, because 'TryStartIOWorkers' will
+    // start I/O workers according to the number of pending tasks.
+    state.pending_io_tasks.push(callback);
+    TryStartIOWorkers(Language::PYTHON, state);
+  } else {
+    auto io_worker = state.idle_io_workers.front();
+    state.idle_io_workers.pop();
+    callback(io_worker);
+  }
+}
+
 void WorkerPool::PushWorker(const std::shared_ptr<WorkerInterface> &worker) {
   // Since the worker is now idle, unset its assigned task ID.
   RAY_CHECK(worker->GetAssignedTaskId().IsNil())
       << "Idle workers cannot have an assigned task ID";
   auto &state = GetStateForLanguage(worker->GetLanguage());
-
   auto it = state.dedicated_workers_to_tasks.find(worker->GetProcess());
   if (it != state.dedicated_workers_to_tasks.end()) {
     // The worker is used for the actor creation task with dynamic options.
@@ -553,8 +608,8 @@ std::shared_ptr<WorkerInterface> WorkerPool::PopWorker(
     } else if (!HasPendingWorkerForTask(task_spec.GetLanguage(), task_spec.TaskId())) {
       // We are not pending a registration from a worker for this task,
       // so start a new worker process for this task.
-      proc = StartWorkerProcess(task_spec.GetLanguage(), task_spec.JobId(),
-                                task_spec.DynamicWorkerOptions());
+      proc = StartWorkerProcess(task_spec.GetLanguage(), rpc::WorkerType::WORKER,
+                                task_spec.JobId(), task_spec.DynamicWorkerOptions());
       if (proc.IsValid()) {
         state.dedicated_workers_to_tasks[proc] = task_spec.TaskId();
         state.tasks_to_dedicated_workers[task_spec.TaskId()] = proc;
@@ -569,7 +624,8 @@ std::shared_ptr<WorkerInterface> WorkerPool::PopWorker(
       } else {
         // There are no more non-actor workers available to execute this task.
         // Start a new worker process.
-        proc = StartWorkerProcess(task_spec.GetLanguage(), JobID::Nil());
+        proc = StartWorkerProcess(task_spec.GetLanguage(), rpc::WorkerType::WORKER,
+                                  JobID::Nil());
       }
     } else {
       // Find an available worker which is already assigned to this job.
@@ -584,7 +640,8 @@ std::shared_ptr<WorkerInterface> WorkerPool::PopWorker(
       if (worker == nullptr) {
         // There are no more non-actor workers available to execute this task.
         // Start a new worker process.
-        proc = StartWorkerProcess(task_spec.GetLanguage(), task_spec.JobId());
+        proc = StartWorkerProcess(task_spec.GetLanguage(), rpc::WorkerType::WORKER,
+                                  task_spec.JobId());
       }
     }
   } else {
@@ -715,6 +772,32 @@ bool WorkerPool::HasPendingWorkerForTask(const Language &language,
   return it != state.tasks_to_dedicated_workers.end();
 }
 
+void WorkerPool::TryStartIOWorkers(const Language &language, State &state) {
+  if (language != Language::PYTHON) {
+    return;
+  }
+  int available_io_workers_num =
+      state.num_starting_io_workers + state.registered_io_workers.size();
+  int max_workers_to_start =
+      RayConfig::instance().max_io_workers() - available_io_workers_num;
+  // Compare first to prevent unsigned underflow.
+  if (state.pending_io_tasks.size() > state.idle_io_workers.size()) {
+    int expected_workers_num =
+        state.pending_io_tasks.size() - state.idle_io_workers.size();
+    if (expected_workers_num > max_workers_to_start) {
+      expected_workers_num = max_workers_to_start;
+    }
+    for (; expected_workers_num > 0; expected_workers_num--) {
+      Process proc = StartWorkerProcess(ray::Language::PYTHON,
+                                        ray::rpc::WorkerType::IO_WORKER, JobID::Nil());
+      if (!proc.IsValid()) {
+        // We may hit the maximum worker start up concurrency limit. Stop.
+        return;
+      }
+    }
+  }
+}
+
 std::string WorkerPool::DebugString() const {
   std::stringstream result;
   result << "WorkerPool:";
diff --git a/src/ray/raylet/worker_pool.h b/src/ray/raylet/worker_pool.h
index 59338db30..62988752c 100644
--- a/src/ray/raylet/worker_pool.h
+++ b/src/ray/raylet/worker_pool.h
@@ -162,6 +162,19 @@ class WorkerPool : public WorkerPoolInterface {
   /// \param The driver to disconnect. The driver must be registered.
   void DisconnectDriver(const std::shared_ptr<WorkerInterface> &driver);
 
+  /// Add an idle I/O worker to the pool.
+  ///
+  /// \param worker The idle I/O worker to add.
+  void PushIOWorker(const std::shared_ptr<WorkerInterface> &worker);
+
+  /// Pop an idle I/O worker from the pool and trigger a callback when
+  /// an I/O worker is available.
+  /// The caller is responsible for pushing the worker back onto the
+  /// pool once the worker has completed its work.
+  ///
+  /// \param callback The callback that returns an available I/O worker.
+  void PopIOWorker(std::function<void(std::shared_ptr<WorkerInterface>)> callback);
+
   /// Add an idle worker to the pool.
   ///
   /// \param The idle worker to add.
@@ -229,11 +242,13 @@ class WorkerPool : public WorkerPoolInterface {
   /// any workers.
   ///
   /// \param language Which language this worker process should be.
+  /// \param worker_type The type of the worker.
   /// \param job_id The ID of the job to which the started worker process belongs.
   /// \param dynamic_options The dynamic options that we should add for worker command.
   /// \return The id of the process that we started if it's positive,
   /// otherwise it means we didn't start a process.
-  Process StartWorkerProcess(const Language &language, const JobID &job_id,
+  Process StartWorkerProcess(const Language &language, const rpc::WorkerType worker_type,
+                             const JobID &job_id,
                              std::vector<std::string> dynamic_options = {});
 
   /// The implementation of how to start a new worker process with command arguments.
@@ -263,6 +278,15 @@ class WorkerPool : public WorkerPoolInterface {
     std::unordered_set<std::shared_ptr<WorkerInterface>> idle;
     /// The pool of idle actor workers.
     std::unordered_map<ActorID, std::shared_ptr<WorkerInterface>> idle_actor;
+    /// The pool of idle I/O workers.
+    std::queue<std::shared_ptr<WorkerInterface>> idle_io_workers;
+    /// The queue of pending I/O tasks.
+    std::queue<std::function<void(std::shared_ptr<WorkerInterface>)>> pending_io_tasks;
+    /// All I/O workers that have registered and are still connected, including both
+    /// idle and executing.
+    std::unordered_set<std::shared_ptr<WorkerInterface>> registered_io_workers;
+    /// Number of starting I/O workers.
+    int num_starting_io_workers = 0;
     /// All workers that have registered and are still connected, including both
     /// idle and executing.
     std::unordered_set<std::shared_ptr<WorkerInterface>> registered_workers;
@@ -306,7 +330,8 @@ class WorkerPool : public WorkerPoolInterface {
   /// (due to worker process crash or any other reasons), remove them
   /// from `starting_worker_processes`. Otherwise if we'll mistakenly
   /// think there are unregistered workers, and won't start new workers.
-  void MonitorStartingWorkerProcess(const Process &proc, const Language &language);
+  void MonitorStartingWorkerProcess(const Process &proc, const Language &language,
+                                    const rpc::WorkerType worker_type);
 
   /// Get the next unallocated port in the free ports list. If a port range isn't
   /// configured, returns 0.
@@ -320,6 +345,13 @@ class WorkerPool : public WorkerPoolInterface {
   /// \param[in] port The port to mark as free.
   void MarkPortAsFree(int port);
 
+  /// Try start all I/O workers waiting to be started.
+  /// \param language The language of the I/O worker. Currently only Python I/O
+  /// workers are effective.
+  /// \param state The state including the number of I/O workers waiting to be
+  /// started.
+  void TryStartIOWorkers(const Language &language, State &state);
+
   /// For Process class for managing subprocesses (e.g. reaping zombies).
   boost::asio::io_service *io_service_;
   /// The maximum number of worker processes that can be started concurrently.
diff --git a/src/ray/raylet/worker_pool_test.cc b/src/ray/raylet/worker_pool_test.cc
index 1cf777821..2d799f257 100644
--- a/src/ray/raylet/worker_pool_test.cc
+++ b/src/ray/raylet/worker_pool_test.cc
@@ -119,7 +119,8 @@ class WorkerPoolTest : public ::testing::TestWithParam<bool> {
         ClientConnection::Create(client_handler, message_handler, std::move(socket),
                                  "worker", {}, error_message_type_);
     std::shared_ptr<Worker> worker_ = std::make_shared<Worker>(
-        WorkerID::FromRandom(), language, "127.0.0.1", client, client_call_manager_);
+        WorkerID::FromRandom(), language, rpc::WorkerType::WORKER, "127.0.0.1", client,
+        client_call_manager_);
     std::shared_ptr<WorkerInterface> worker =
         std::dynamic_pointer_cast<WorkerInterface>(worker_);
     worker->AssignJobId(job_id);
@@ -155,7 +156,7 @@ class WorkerPoolTest : public ::testing::TestWithParam<bool> {
                 static_cast<int>(desired_initial_worker_process_count));
     Process last_started_worker_process;
     for (int i = 0; i < desired_initial_worker_process_count; i++) {
-      worker_pool_->StartWorkerProcess(language, JOB_ID);
+      worker_pool_->StartWorkerProcess(language, rpc::WorkerType::WORKER, JOB_ID);
       ASSERT_TRUE(worker_pool_->NumWorkerProcessesStarting() <=
                   expected_worker_process_count);
       Process prev = worker_pool_->LastStartedWorkerProcess();
@@ -233,7 +234,8 @@ TEST_P(WorkerPoolTest, CompareWorkerProcessObjects) {
 }
 
 TEST_P(WorkerPoolTest, HandleWorkerRegistration) {
-  Process proc = worker_pool_->StartWorkerProcess(Language::JAVA, JOB_ID);
+  Process proc =
+      worker_pool_->StartWorkerProcess(Language::JAVA, rpc::WorkerType::WORKER, JOB_ID);
   std::vector<std::shared_ptr<WorkerInterface>> workers;
   for (int i = 0; i < NUM_WORKERS_PER_PROCESS_JAVA; i++) {
     workers.push_back(CreateWorker(Process(), Language::JAVA));
@@ -361,7 +363,7 @@ TEST_P(WorkerPoolTest, StartWorkerWithDynamicOptionsCommand) {
   TaskSpecification task_spec = ExampleTaskSpec(
       ActorID::Nil(), Language::JAVA, JOB_ID,
       ActorID::Of(JOB_ID, TaskID::ForDriverTask(JOB_ID), 1), {"test_op_0", "test_op_1"});
-  worker_pool_->StartWorkerProcess(Language::JAVA, JOB_ID,
+  worker_pool_->StartWorkerProcess(Language::JAVA, rpc::WorkerType::WORKER, JOB_ID,
                                    task_spec.DynamicWorkerOptions());
   const auto real_command =
       worker_pool_->GetWorkerCommand(worker_pool_->LastStartedWorkerProcess());
diff --git a/src/ray/raylet_client/raylet_client.cc b/src/ray/raylet_client/raylet_client.cc
index e9ff24a0c..545569280 100644
--- a/src/ray/raylet_client/raylet_client.cc
+++ b/src/ray/raylet_client/raylet_client.cc
@@ -80,9 +80,9 @@ raylet::RayletClient::RayletClient(
 raylet::RayletClient::RayletClient(
     boost::asio::io_service &io_service,
     std::shared_ptr<ray::rpc::NodeManagerWorkerClient> grpc_client,
-    const std::string &raylet_socket, const WorkerID &worker_id, bool is_worker,
-    const JobID &job_id, const Language &language, const std::string &ip_address,
-    ClientID *raylet_id, int *port,
+    const std::string &raylet_socket, const WorkerID &worker_id,
+    rpc::WorkerType worker_type, const JobID &job_id, const Language &language,
+    const std::string &ip_address, ClientID *raylet_id, int *port,
     std::unordered_map<std::string, std::string> *internal_config,
     const std::string &job_config)
     : grpc_client_(std::move(grpc_client)),
@@ -94,9 +94,11 @@ raylet::RayletClient::RayletClient(
       new raylet::RayletConnection(io_service, raylet_socket, -1, -1));
 
   flatbuffers::FlatBufferBuilder fbb;
+  // TODO(suquark): Use `WorkerType` in `common.proto` without converting to int.
   auto message = protocol::CreateRegisterClientRequest(
-      fbb, is_worker, to_flatbuf(fbb, worker_id), getpid(), to_flatbuf(fbb, job_id),
-      language, fbb.CreateString(ip_address), /*port=*/0, fbb.CreateString(job_config_));
+      fbb, static_cast<int>(worker_type), to_flatbuf(fbb, worker_id), getpid(),
+      to_flatbuf(fbb, job_id), language, fbb.CreateString(ip_address), /*port=*/0,
+      fbb.CreateString(job_config_));
   fbb.Finish(message);
   // Register the process ID with the raylet.
   // NOTE(swang): If raylet exits and we are registered as a worker, we will get killed.
@@ -308,6 +310,38 @@ void raylet::RayletClient::RequestWorkerLease(
   grpc_client_->RequestWorkerLease(request, callback);
 }
 
+/// Spill objects to external storage.
+/// \param object_ids The IDs of objects to be spilled.
+Status raylet::RayletClient::ForceSpillObjects(const std::vector<ObjectID> &object_ids) {
+  flatbuffers::FlatBufferBuilder fbb;
+  auto message =
+      protocol::CreateForceSpillObjectsRequest(fbb, to_flatbuf(fbb, object_ids));
+  fbb.Finish(message);
+  std::vector<uint8_t> reply;
+  RAY_RETURN_NOT_OK(conn_->AtomicRequestReply(MessageType::ForceSpillObjectsRequest,
+                                              MessageType::ForceSpillObjectsReply, &reply,
+                                              &fbb));
+  RAY_UNUSED(flatbuffers::GetRoot<protocol::ForceSpillObjectsReply>(reply.data()));
+  return Status::OK();
+}
+
+/// Restore spilled objects from external storage.
+/// \param object_ids The IDs of objects to be restored.
+Status raylet::RayletClient::ForceRestoreSpilledObjects(
+    const std::vector<ObjectID> &object_ids) {
+  flatbuffers::FlatBufferBuilder fbb;
+  auto message =
+      protocol::CreateForceRestoreSpilledObjectsRequest(fbb, to_flatbuf(fbb, object_ids));
+  fbb.Finish(message);
+  std::vector<uint8_t> reply;
+  RAY_RETURN_NOT_OK(conn_->AtomicRequestReply(
+      MessageType::ForceRestoreSpilledObjectsRequest,
+      MessageType::ForceRestoreSpilledObjectsReply, &reply, &fbb));
+  RAY_UNUSED(
+      flatbuffers::GetRoot<protocol::ForceRestoreSpilledObjectsReply>(reply.data()));
+  return Status::OK();
+}
+
 Status raylet::RayletClient::ReturnWorker(int worker_port, const WorkerID &worker_id,
                                           bool disconnect_worker) {
   rpc::ReturnWorkerRequest request;
diff --git a/src/ray/raylet_client/raylet_client.h b/src/ray/raylet_client/raylet_client.h
index 78421f43a..ddf80c6ea 100644
--- a/src/ray/raylet_client/raylet_client.h
+++ b/src/ray/raylet_client/raylet_client.h
@@ -23,6 +23,7 @@
 #include "ray/common/status.h"
 #include "ray/common/task/task_spec.h"
 #include "ray/rpc/node_manager/node_manager_client.h"
+#include "src/ray/protobuf/common.pb.h"
 #include "src/ray/protobuf/gcs.pb.h"
 
 using ray::ActorCheckpointID;
@@ -162,7 +163,7 @@ class RayletClient : public PinObjectsInterface,
   /// \param grpc_client gRPC client to the raylet.
   /// \param raylet_socket The name of the socket to use to connect to the raylet.
   /// \param worker_id A unique ID to represent the worker.
-  /// \param is_worker Whether this client is a worker. If it is a worker, an
+  /// \param worker_type The type of the worker. If it is a certain worker type, an
   /// additional message will be sent to register as one.
   /// \param job_id The ID of the driver. This is non-nil if the client is a driver.
   /// \param language Language of the worker.
@@ -175,7 +176,7 @@ class RayletClient : public PinObjectsInterface,
   RayletClient(boost::asio::io_service &io_service,
                std::shared_ptr<ray::rpc::NodeManagerWorkerClient> grpc_client,
                const std::string &raylet_socket, const WorkerID &worker_id,
-               bool is_worker, const JobID &job_id, const Language &language,
+               rpc::WorkerType worker_type, const JobID &job_id, const Language &language,
                const std::string &ip_address, ClientID *raylet_id, int *port,
                std::unordered_map<std::string, std::string> *internal_config,
                const std::string &job_config);
@@ -318,6 +319,16 @@ class RayletClient : public PinObjectsInterface,
   ray::Status SetResource(const std::string &resource_name, const double capacity,
                           const ray::ClientID &client_Id);
 
+  /// Spill objects to external storage.
+  /// \param object_ids The IDs of objects to be spilled.
+  /// \return ray::Status
+  ray::Status ForceSpillObjects(const std::vector<ObjectID> &object_ids);
+
+  /// Restore spilled objects from external storage.
+  /// \param object_ids The IDs of objects to be restored.
+  /// \return ray::Status
+  ray::Status ForceRestoreSpilledObjects(const std::vector<ObjectID> &object_ids);
+
   /// Implements WorkerLeaseInterface.
   void RequestWorkerLease(
       const ray::TaskSpecification &resource_spec,
diff --git a/src/ray/rpc/worker/core_worker_client.h b/src/ray/rpc/worker/core_worker_client.h
index f30046fa5..cfb68df93 100644
--- a/src/ray/rpc/worker/core_worker_client.h
+++ b/src/ray/rpc/worker/core_worker_client.h
@@ -175,6 +175,13 @@ class CoreWorkerClientInterface {
                                  const ClientCallback<WaitForRefRemovedReply> &callback) {
   }
 
+  virtual void SpillObjects(const SpillObjectsRequest &request,
+                            const ClientCallback<SpillObjectsReply> &callback) {}
+
+  virtual void RestoreSpilledObjects(
+      const RestoreSpilledObjectsRequest &request,
+      const ClientCallback<RestoreSpilledObjectsReply> &callback) {}
+
   virtual void PlasmaObjectReady(const PlasmaObjectReadyRequest &request,
                                  const ClientCallback<PlasmaObjectReadyReply> &callback) {
   }
@@ -231,6 +238,10 @@ class CoreWorkerClient : public std::enable_shared_from_this<CoreWorkerClient>,
 
   VOID_RPC_CLIENT_METHOD(CoreWorkerService, WaitForRefRemoved, grpc_client_, override)
 
+  VOID_RPC_CLIENT_METHOD(CoreWorkerService, SpillObjects, grpc_client_, override)
+
+  VOID_RPC_CLIENT_METHOD(CoreWorkerService, RestoreSpilledObjects, grpc_client_, override)
+
   VOID_RPC_CLIENT_METHOD(CoreWorkerService, PlasmaObjectReady, grpc_client_, override)
 
   void PushActorTask(std::unique_ptr<PushTaskRequest> request, bool skip_queue,
diff --git a/src/ray/rpc/worker/core_worker_server.h b/src/ray/rpc/worker/core_worker_server.h
index cf0c0e519..4926e047f 100644
--- a/src/ray/rpc/worker/core_worker_server.h
+++ b/src/ray/rpc/worker/core_worker_server.h
@@ -41,6 +41,8 @@ namespace rpc {
   RPC_SERVICE_HANDLER(CoreWorkerService, RemoteCancelTask)               \
   RPC_SERVICE_HANDLER(CoreWorkerService, GetCoreWorkerStats)             \
   RPC_SERVICE_HANDLER(CoreWorkerService, LocalGC)                        \
+  RPC_SERVICE_HANDLER(CoreWorkerService, SpillObjects)                   \
+  RPC_SERVICE_HANDLER(CoreWorkerService, RestoreSpilledObjects)          \
   RPC_SERVICE_HANDLER(CoreWorkerService, PlasmaObjectReady)
 
 #define RAY_CORE_WORKER_DECLARE_RPC_HANDLERS                              \
@@ -58,6 +60,8 @@ namespace rpc {
   DECLARE_VOID_RPC_SERVICE_HANDLER_METHOD(RemoteCancelTask)               \
   DECLARE_VOID_RPC_SERVICE_HANDLER_METHOD(GetCoreWorkerStats)             \
   DECLARE_VOID_RPC_SERVICE_HANDLER_METHOD(LocalGC)                        \
+  DECLARE_VOID_RPC_SERVICE_HANDLER_METHOD(SpillObjects)                   \
+  DECLARE_VOID_RPC_SERVICE_HANDLER_METHOD(RestoreSpilledObjects)          \
   DECLARE_VOID_RPC_SERVICE_HANDLER_METHOD(PlasmaObjectReady)
 
 /// Interface of the `CoreWorkerServiceHandler`, see `src/ray/protobuf/core_worker.proto`.