---
layout: docs
page_title: Storage Plugins
description: Learn how Nomad manages dynamic storage plugins.
---

# Storage Plugins

Nomad has built-in support for scheduling compute resources such as
CPU, memory, and networking. Nomad's storage plugin support extends
this to allow scheduling tasks with externally created storage
volumes. Storage plugins are third-party plugins that conform to the
[Container Storage Interface (CSI)][csi-spec] specification.

Storage plugins are created dynamically as Nomad jobs, unlike device
and task driver plugins that need to be installed and configured on
each client. Each dynamic plugin type has its own type-specific job
spec block; currently there is only the `csi_plugin` type. Nomad
tracks which clients have instances of a given plugin, and
communicates with plugins over a Unix domain socket that it creates
inside the plugin's tasks.

## CSI Plugins

Every storage vendor has its own APIs and workflows, and the
industry-standard Container Storage Interface specification unifies
these APIs in a way that's agnostic to both the storage vendor and the
container orchestrator. Each storage provider can build its own CSI
plugin. Jobs can claim storage volumes from AWS Elastic Block Storage
(EBS) volumes, GCP persistent disks, Ceph, Portworx, vSphere, etc. The
Nomad scheduler will be aware of volumes created by CSI plugins and
schedule workloads based on the availability of volumes on a given
Nomad client node. A list of available CSI plugins can be found in the
[Kubernetes CSI documentation][csi-drivers-list]. Any of these plugins
should work with Nomad out of the box.

A CSI plugin task requires the [`csi_plugin`][csi_plugin] block:

```hcl
csi_plugin {
  id        = "csi-hostpath"
  type      = "monolith"
  mount_dir = "/csi"
}
```

There are three **types** of CSI plugins. **Controller Plugins**
communicate with the storage provider's APIs. For example, for a job
that needs an AWS EBS volume, Nomad will tell the controller plugin
that it needs a volume to be "published" to the client node, and the
controller will make the API calls to AWS to attach the EBS volume to
the right EC2 instance. **Node Plugins** do the work on each client
node, like creating mount points. **Monolith Plugins** are plugins
that perform both the controller and node roles in the same
instance. Not every plugin provider has or needs a controller; that's
specific to the provider implementation.

Plugins mount and unmount volumes but are not in the data path once
the volume is mounted for a task. Plugin tasks are needed when tasks
using their volumes stop, so plugins should be left running on a Nomad
client until all tasks using their volumes are stopped. The `nomad
node drain` command handles this automatically by stopping plugin
tasks last.

Typically, you should run node plugins as Nomad `system` jobs so they
can mount volumes on any client where they are running. Controller
plugins can create and attach volumes anywhere they can communicate
with the storage provider's API, so they can usually be run as
`service` jobs. You should always run more than one controller plugin
allocation for high availability.

Nomad exposes a Unix domain socket named `csi.sock` inside each CSI
plugin task, and communicates over the gRPC protocol expected by the
CSI specification. The `mount_dir` field tells Nomad where the plugin
expects to find the socket file. The path to this socket is exposed in
the container as the `CSI_ENDPOINT` environment variable.

### Plugin Lifecycle and State

CSI plugins report their health like other Nomad jobs. If the plugin
crashes or otherwise terminates, Nomad will launch it again using the
same `restart` and `reschedule` logic used for other jobs. If plugins
are unhealthy, Nomad will mark the volumes they manage as
"unscheduable".

Storage plugins don't have any responsibility (or ability) to monitor
the state of tasks that claim their volumes. Nomad sends mount and
publish requests to storage plugins when a task claims a volume, and
unmount and unpublish requests when a task stops.

The dynamic plugin registry persists state to the Nomad client so that
it can restore volume managers for plugin jobs after client restarts
without disrupting storage.

### Volume Lifecycle

The Nomad scheduler decides whether a given client can run an
allocation based on whether it has a node plugin present for the
volume. But before a task can use a volume the client needs to "claim"
the volume for the allocation. The client makes an RPC call to the
server and waits for a response; the allocation's tasks won't start
until the volume has been claimed and is ready.

If the volume's plugin requires a controller, the server will send an
RPC to any Nomad client where that controller is running. The Nomad
client will forward this request over the controller plugin's gRPC
socket. The controller plugin will make the request volume available
to the node that needs it.

Once the controller is done (or if there's no controller required),
the server will increment the count of claims on the volume and return
to the client. This count passes through Nomad's state store so that
Nomad has a consistent view of which volumes are available for
scheduling.

The client then makes RPC calls to the node plugin running on that
client, and the node plugin mounts the volume to a staging area in
the Nomad data directory. Nomad will bind-mount this staged directory
into each task that mounts the volume.

This cycle is reversed when a task that claims a volume becomes
terminal. The client frees the volume locally by making "unpublish"
RPCs to the node plugin. The node plugin unmounts the bind-mount from
the allocation and unmounts the volume from the plugin (if it's not in
use by another task). The client will then send an "unpublish" RPC to
the server, which will forward it to the the controller plugin (if
any), and decrement the claim count for the volume. At this point the
volume’s claim capacity has been freed up for scheduling.

[csi-spec]: https://github.com/container-storage-interface/spec
[csi-drivers-list]: https://kubernetes-csi.github.io/docs/drivers.html
[csi_plugin]: /docs/job-specification/csi_plugin
