kubernetes/docs/user-guide/volumes.md

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<h2>PLEASE NOTE: This document applies to the HEAD of the source tree</h2>

If you are using a released version of Kubernetes, you should
refer to the docs that go with that version.

<strong>
The latest 1.0.x release of this document can be found
[here](http://releases.k8s.io/release-1.0/docs/user-guide/volumes.md).

Documentation for other releases can be found at
[releases.k8s.io](http://releases.k8s.io).
</strong>
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# Volumes

On-disk files in a container are ephemeral, which presents some problems for
non-trivial applications when running in containers.  First, when a container
crashes kubelet will restart it, but the files will be lost - the
container starts with a clean slate.  Second, when running containers together
in a `Pod` it is often necessary to share files between those containers.  The
Kubernetes `Volume` abstraction solves both of these problems.

Familiarity with [pods](pods.md) is suggested.

**Table of Contents**
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- [Volumes](#volumes)
  - [Background](#background)
  - [Types of Volumes](#types-of-volumes)
    - [emptyDir](#emptydir)
    - [hostPath](#hostpath)
    - [gcePersistentDisk](#gcepersistentdisk)
      - [Creating a PD](#creating-a-pd)
      - [Example pod](#example-pod)
    - [awsElasticBlockStore](#awselasticblockstore)
      - [Creating an EBS volume](#creating-an-ebs-volume)
      - [AWS EBS Example configuration](#aws-ebs-example-configuration)
    - [nfs](#nfs)
    - [iscsi](#iscsi)
    - [flocker](#flocker)
    - [glusterfs](#glusterfs)
    - [rbd](#rbd)
    - [gitRepo](#gitrepo)
    - [secret](#secret)
    - [persistentVolumeClaim](#persistentvolumeclaim)
    - [downwardAPI](#downwardapi)
  - [Resources](#resources)

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## Background

Docker also has a concept of
[volumes](https://docs.docker.com/userguide/dockervolumes/), though it is
somewhat looser and less managed.  In Docker, a volume is simply a directory on
disk or in another container.  Lifetimes are not managed and until very
recently there were only local-disk-backed volumes.  Docker now provides volume
drivers, but the functionality is very limited for now (e.g. as of Docker 1.7
only one volume driver is allowed per container and there is no way to pass
parameters to volumes).

A Kubernetes volume, on the other hand, has an explicit lifetime - the same as
the pod that encloses it.  Consequently, a volume outlives any containers that run
within the Pod, and data is preserved across Container restarts. Of course, when a
Pod ceases to exist, the volume will cease to exist, too.  Perhaps more
importantly than this, Kubernetes supports many type of volumes, and a Pod can
use any number of them simultaneously.

At its core, a volume is just a directory, possibly with some data in it, which
is accessible to the containers in a pod.  How that directory comes to be, the
medium that backs it, and the contents of it are determined by the particular
volume type used.

To use a volume, a pod specifies what volumes to provide for the pod (the
[`spec.volumes`](http://kubernetes.io/third_party/swagger-ui/#!/v1/createPod)
field) and where to mount those into containers(the
[`spec.containers.volumeMounts`](http://kubernetes.io/third_party/swagger-ui/#!/v1/createPod)
field).

A process in a container sees a filesystem view composed from their Docker
image and volumes.  The [Docker
image](https://docs.docker.com/userguide/dockerimages/) is at the root of the
filesystem hierarchy, and any volumes are mounted at the specified paths within
the image.  Volumes can not mount onto other volumes or have hard links to
other volumes.  Each container in the Pod must independently specify where to
mount each volume.

## Types of Volumes

Kubernetes supports several types of Volumes:
   * `emptyDir`
   * `hostPath`
   * `gcePersistentDisk`
   * `awsElasticBlockStore`
   * `nfs`
   * `iscsi`
   * `flocker`
   * `glusterfs`
   * `rbd`
   * `gitRepo`
   * `secret`
   * `persistentVolumeClaim`

We welcome additional contributions.

### emptyDir

An `emptyDir` volume is first created when a Pod is assigned to a Node, and
exists as long as that Pod is running on that node.  As the name says, it is
initially empty.  Containers in the pod can all read and write the same
files in the `emptyDir` volume, though that volume can be mounted at the same
or different paths in each container.  When a Pod is removed from a node for
any reason, the data in the `emptyDir` is deleted forever.  NOTE: a container
crashing does *NOT* remove a pod from a node, so the data in an `emptyDir`
volume is safe across container crashes.

Some uses for an `emptyDir` are:

* scratch space, such as for a disk-based mergesortcw
* checkpointing a long computation for recovery from crashes
* holding files that a content-manager container fetches while a webserver
  container serves the data

By default, `emptyDir` volumes are stored on whatever medium is backing the
machine - that might be disk or SSD or network storage, depending on your
environment.  However, you can set the `emptyDir.medium` field to `"Memory"`
to tell Kubernetes to mount a tmpfs (RAM-backed filesystem) for you instead.
While tmpfs is very fast, be aware that unlike disks, tmpfs is cleared on
machine reboot and any files you write will count against your container's
memory limit.

### hostPath

A `hostPath` volume mounts a file or directory from the host node's filesystem
into your pod.  This is not something that most Pods will need, but it offers a
powerful escape hatch for some applications.

For example, some uses for a `hostPath` are:

* running a container that needs access to Docker internals; use a `hostPath`
  of `/var/lib/docker`
* running cAdvisor in a container; use a `hostPath` of `/dev/cgroups`

Watch out when using this type of volume, because:

* pods with identical configuration (such as created from a podTemplate) may
  behave differently on different nodes due to different files on the nodes
* when Kubernetes adds resource-aware scheduling, as is planned, it will not be
  able to account for resources used by a `hostPath`

### gcePersistentDisk

A `gcePersistentDisk` volume mounts a Google Compute Engine (GCE) [Persistent
Disk](http://cloud.google.com/compute/docs/disks) into your pod.  Unlike
`emptyDir`, which is erased when a Pod is removed, the contents of a PD are
preserved and the volume is merely unmounted.  This means that a PD can be
pre-populated with data, and that data can be "handed off" between pods.

__Important: You must create a PD using `gcloud` or the GCE API or UI
before you can use it__

There are some restrictions when using a `gcePersistentDisk`:

* the nodes on which pods are running must be GCE VMs
* those VMs need to be in the same GCE project and zone as the PD

A feature of PD is that they can be mounted as read-only by multiple consumers
simultaneously.  This means that you can pre-populate a PD with your dataset
and then serve it in parallel from as many pods as you need.  Unfortunately,
PDs can only be mounted by a single consumer in read-write mode - no
simultaneous readers allowed.

Using a PD on a pod controlled by a ReplicationController will fail unless
the PD is read-only or the replica count is 0 or 1.

#### Creating a PD

Before you can use a GCE PD with a pod, you need to create it.

```sh
gcloud compute disks create --size=500GB --zone=us-central1-a my-data-disk
```

#### Example pod

```yaml
apiVersion: v1
kind: Pod
metadata:
  name: test-pd
spec:
  containers:
  - image: gcr.io/google_containers/test-webserver
    name: test-container
    volumeMounts:
    - mountPath: /test-pd
      name: test-volume
  volumes:
  - name: test-volume
    # This GCE PD must already exist.
    gcePersistentDisk:
      pdName: my-data-disk
      fsType: ext4
```

### awsElasticBlockStore

An `awsElasticBlockStore` volume mounts an Amazon Web Services (AWS) [EBS
Volume](http://aws.amazon.com/ebs/) into your pod.  Unlike
`emptyDir`, which is erased when a Pod is removed, the contents of an EBS
volume are preserved and the volume is merely unmounted.  This means that an
EBS volume can be pre-populated with data, and that data can be "handed off"
between pods.

__Important: You must create an EBS volume using `aws ec2 create-volume` or
the AWS API before you can use it__

There are some restrictions when using an awsElasticBlockStore volume:

* the nodes on which pods are running must be AWS EC2 instances
* those instances need to be in the same region and availability-zone as the EBS volume
* EBS only supports a single EC2 instance mounting a volume

#### Creating an EBS volume

Before you can use a EBS volume with a pod, you need to create it.

```sh
aws ec2 create-volume --availability-zone eu-west-1a --size 10 --volume-type gp2
```

Make sure the zone matches the zone you brought up your cluster in.  (And also check that the size and EBS volume
type are suitable for your use!)

#### AWS EBS Example configuration

```yaml
apiVersion: v1
kind: Pod
metadata:
  name: test-ebs
spec:
  containers:
  - image: gcr.io/google_containers/test-webserver
    name: test-container
    volumeMounts:
    - mountPath: /test-ebs
      name: test-volume
  volumes:
  - name: test-volume
    # This AWS EBS volume must already exist.
    awsElasticBlockStore:
      volumeID: aws://<availability-zone>/<volume-id>
      fsType: ext4
```

(Note: the syntax of volumeID is currently awkward; #10181 fixes it)

### nfs

An `nfs` volume allows an existing NFS (Network File System) share to be
mounted into your pod. Unlike `emptyDir`, which is erased when a Pod is
removed, the contents of an `nfs` volume are preserved and the volume is merely
unmounted.  This means that an NFS volume can be pre-populated with data, and
that data can be "handed off" between pods.  NFS can be mounted by multiple
writers simultaneously.

__Important: You must have your own NFS server running with the share exported
before you can use it__

See the [NFS example](../../examples/nfs/) for more details.

For example, [this file](../../examples/nfs/nfs-web-pod.yaml) demonstrates how to
specify the usage of an NFS volume within a pod.

In this example one can see that a `volumeMount` called `nfs` is being mounted
onto `/var/www/html` in the container `web`.  The volume "nfs" is defined as
type `nfs`, with the NFS server serving from `nfs-server.default.kube.local`
and exporting directory `/` as the share.  The mount being created in this
example is writeable.

### iscsi

An `iscsi` volume allows an existing iSCSI (SCSI over IP) volume to be mounted
into your pod.  Unlike `emptyDir`, which is erased when a Pod is removed, the
contents of an `iscsi` volume are preserved and the volume is merely
unmounted.  This means that an iscsi volume can be pre-populated with data, and
that data can be "handed off" between pods.

__Important: You must have your own iSCSI server running with the volume
created before you can use it__

A feature of iSCSI is that it can be mounted as read-only by multiple consumers
simultaneously.  This means that you can pre-populate a volume with your dataset
and then serve it in parallel from as many pods as you need.  Unfortunately,
iSCSI volumes can only be mounted by a single consumer in read-write mode - no
simultaneous readers allowed.

See the [iSCSI example](../../examples/iscsi/) for more details.

### flocker

[Flocker](https://clusterhq.com/flocker) is an open-source clustered container data volume manager. It provides management
and orchestration of data volumes backed by a variety of storage backends.

A `flocker` volume allows a Flocker dataset to be mounted into a pod. If the
dataset does not already exist in Flocker, it needs to be created with Flocker
CLI or the using the Flocker API. If the dataset already exists it will
reattached by Flocker to the node that the pod is scheduled. This means data
can be "handed off" between pods as required.

__Important: You must have your own Flocker installation running before you can use it__

See the [Flocker example](../../examples/flocker/) for more details.

### glusterfs

A `glusterfs` volume allows a [Glusterfs](http://www.gluster.org) (an open
source networked filesystem) volume to be mounted into your pod.  Unlike
`emptyDir`, which is erased when a Pod is removed, the contents of a
`glusterfs` volume are preserved and the volume is merely unmounted.  This
means that a glusterfs volume can be pre-populated with data, and that data can
be "handed off" between pods.  GlusterFS can be mounted by multiple writers
simultaneously.

__Important: You must have your own GlusterFS installation running before you
can use it__

See the [GlusterFS example](../../examples/glusterfs/) for more details.

### rbd

An `rbd` volume allows a [Rados Block
Device](http://ceph.com/docs/master/rbd/rbd/) volume to be mounted into your
pod.  Unlike `emptyDir`, which is erased when a Pod is removed, the contents of
a `rbd` volume are preserved and the volume is merely unmounted.  This
means that a RBD volume can be pre-populated with data, and that data can
be "handed off" between pods.

__Important: You must have your own Ceph installation running before you
can use RBD__

A feature of RBD is that it can be mounted as read-only by multiple consumers
simultaneously.  This means that you can pre-populate a volume with your dataset
and then serve it in parallel from as many pods as you need.  Unfortunately,
RBD volumes can only be mounted by a single consumer in read-write mode - no
simultaneous writers allowed.

See the [RBD example](../../examples/rbd/) for more details.

### gitRepo

A `gitRepo` volume is an example of what can be done as a volume plugin.  It
mounts an empty directory and clones a git repository into it for your pod to
use.  In the future, such volumes may be moved to an even more decoupled model,
rather than extending the Kubernetes API for every such use case.

### secret

A `secret` volume is used to pass sensitive information, such as passwords, to
pods.  You can store secrets in the Kubernetes API and mount them as files for
use by pods without coupling to Kubernetes directly.  `secret` volumes are
backed by tmpfs (a RAM-backed filesystem) so they are never written to
non-volatile storage.

__Important: You must create a secret in the Kubernetes API before you can use
it__

Secrets are described in more detail [here](secrets.md).

### persistentVolumeClaim

A `persistentVolumeClaim` volume is used to mount a
[PersistentVolume](persistent-volumes.md) into a pod.  PersistentVolumes are a
way for users to "claim" durable storage (such as a GCE PersistentDisk or an
iSCSI volume) without knowing the details of the particular cloud environment.

See the [PersistentVolumes example](persistent-volumes/) for more
details.

### downwardAPI

A `downwardAPI` volume is used to make downward API data available to applications.
It mounts a directory and writes the requested data in plain text files.

See the [`downwardAPI` volume example](downward-api/volume/README.md)  for more details.

## Resources

The storage media (Disk, SSD, etc) of an `emptyDir` volume is determined by the
medium of the filesystem holding the kubelet root dir (typically
`/var/lib/kubelet`).  There is no limit on how much space an `emptyDir` or
`hostPath` volume can consume, and no isolation between containers or between
pods.

In the future, we expect that `emptyDir` and `hostPath` volumes will be able to
request a certain amount of space using a [resource](compute-resources.md)
specification, and to select the type of media to use, for clusters that have
several media types.


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