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Merge pull request #1677 from erictune/volume_docs

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Document GCEPersistentDisk.
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Brendan Burns 2014-10-09 14:08:12 -07:00
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docs/volumes.md
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# Volumes # Volumes
This document describes the current state of Volumes in kubernetes. Familiarity with [pods](./pods.md) is suggested. This document describes the current state of Volumes in kubernetes. Familiarity with [pods](./pods.md) is suggested.
A Volume is a directory, possibly with some data in it, which is accessible to a Container. Kubernetes Volumes are similar to but not the same as [Docker Volumes](https://docs.docker.com/userguide/dockervolumes/). A Volume is a directory, possibly with some data in it, which is accessible to a Container. Kubernetes Volumes are similar to but not the same as [Docker Volumes](https://docs.docker.com/userguide/dockervolumes/).
A Pod specifies which Volumes its containers need in its [ContainerManifest](https://developers.google.com/compute/docs/containers/container_vms#container_manifest) property. A Pod specifies which Volumes its containers need in its [ContainerManifest](https://developers.google.com/compute/docs/containers/container_vms#container_manifest) property.
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## Types of Volumes ## Types of Volumes
Kubernetes currently supports two types of Volumes, but more may be added in the future. Kubernetes currently supports three types of Volumes, but more may be added in the future.
### EmptyDir ### EmptyDir
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- pods with identical configuration (such as created from a podTemplate) may behave differently on different nodes due to different files on different nodes. - pods with identical configuration (such as created from a podTemplate) may behave differently on different nodes due to different files on different nodes.
- When Kubernetes adds resource-aware scheduling, as is planned, it will not be able to account for resources used by a HostDir. - When Kubernetes adds resource-aware scheduling, as is planned, it will not be able to account for resources used by a HostDir.
### GCEPersistentDisk
A Volume with a GCEPersistentDisk property allows access to files on a Google Compute Engine (GCE)
[Persistent Disk](http://cloud.google.com/compute/docs/disks).
There are some restrictions when using a GCEPersistentDisk:
- the nodes (what the kubelet runs on) need to be GCE VMs
- those VMs need to be in the same GCE project and zone as the PD
- avoid creating multiple pods that use the same Volume
- if multiple pods refer to the same Volume and both are scheduled on the same machine, regardless of whether they are read-only or read-write, then the second pod scheduled will fail.
- Replication controllers can only be created for pods that use read-only mounts.