---
layout: docs
page_title: volume Block - Job Specification
description: >-
  The "volume" block allows the group to specify that it requires a given
  volume from the cluster. Nomad will automatically handle ensuring that the
  volume is available and mounted into the task.
---

# `volume` Block

<Placement groups={['job', 'group', 'volume']} />

The `volume` block allows the group to specify that it requires a
given volume from the cluster.

The key of the block is the name of the volume as it will be exposed
to task configuration.

```hcl
job "docs" {
  group "example" {
    volume "certs" {
      type      = "host"
      source    = "ca-certificates"
      read_only = true
    }
  }
}
```

```hcl
job "docs" {
  group "example" {
    volume "data" {
      type            = "csi"
      source          = "csi-volume"
      read_only       = true
      attachment_mode = "file-system"
      access_mode     = "single-node-writer"
      per_alloc       = true

      mount_options {
        fs_type     = "ext4"
        mount_flags = ["noatime"]
      }
    }
  }
}
```


The Nomad server will ensure that the allocations are only scheduled
on hosts that have a set of volumes that meet the criteria specified
in the `volume` blocks. These may be [host volumes][host_volume]
configured on the client, or [CSI volumes][csi_volume] dynamically
mounted by [CSI plugins][csi_plugin].

The Nomad client will make the volumes available to tasks according to
the [volume_mount][volume_mount] block in the `task` configuration.

## `volume` Parameters

- `type` `(string: "")` - Specifies the type of a given volume. The
  valid volume types are `"host"` and `"csi"`.

- `source` `(string: <required>)` - The name of the volume to
  request. When using `host_volume`'s this should match the published
  name of the host volume. When using `csi` volumes, this should match
  the ID of the registered volume.

- `read_only` `(bool: false)` - Specifies that the group only requires
  read only access to a volume and is used as the default value for
  the `volume_mount -> read_only` configuration. This value is also
  used for validating `host_volume` ACLs and for scheduling when a
  matching `host_volume` requires `read_only` usage.

- `per_alloc` `(bool: false)` - Specifies that the `source` of the volume
  should have the suffix `[n]`, where `n` is the allocation index. This allows
  mounting a unique volume per allocation, so long as the volume's source is
  named appropriately. For example, with the source `myvolume` and `per_alloc
  = true`, the allocation named `myjob.mygroup.mytask[0]` will require a
  volume ID `myvolume[0]`.

  The `per_alloc` field cannot be true for system jobs, sysbatch jobs, or jobs
  that use canaries.

The following fields are only valid for volumes with `type = "csi"`:

- `access_mode` `(string: <required>)` - Defines whether a volume should be
  available concurrently. The `access_mode` and `attachment_mode` together
  must exactly match one of the volume's `capability` blocks. Can be one of
  `"single-node-reader-only"`, `"single-node-writer"`,
  `"multi-node-reader-only"`, `"multi-node-single-writer"`, or
  `"multi-node-multi-writer"`. Most CSI plugins support only single-node
  modes. Consult the documentation of the storage provider and CSI plugin.

- `attachment_mode` `(string: <required>)` - The storage API that will be used
  by the volume. The `access_mode` and `attachment_mode` together must exactly
  match one of the volume's `capability` blocks. Most storage providers will
  support `"file-system"`, to mount volumes using the CSI filesystem API. Some
  storage providers will support `"block-device"`, which will mount the volume
  with the CSI block device API within the container.

- `mount_options` - Options for mounting CSI volumes that have the
  `file-system` [attachment mode]. These options override the `mount_options`
  field from [volume registration]. Consult the documentation for your storage
  provider and CSI plugin as to whether these options are required or
  necessary.

  - `fs_type`: file system type (ex. `"ext4"`)
  - `mount_flags`: the flags passed to `mount` (ex. `["ro", "noatime"]`)

## Volume Interpolation

Because volumes represent state, many workloads with multiple allocations will
want to mount specific volumes to specific tasks. The `volume` block is used
to schedule workloads, so `${NOMAD_ALLOC_INDEX}` can't be used directly in the
`volume.source` field. The following job specification demonstrates how to use
multiple volumes with multiple allocations, using the `per_alloc` field. This
job specification also shows using HCL2 -variables interpolation to expose
information to the task's environment.

```hcl
variables {
  path = "test"
}

job "example" {
  datacenters = ["dc1"]

  group "cache" {

    count = 2

    volume "cache-volume" {
      type            = "csi"
      source          = "test-volume"
      attachment_mode = "file-system"
      access_mode     = "single-node-writer"
      per_alloc       = true
    }

    network {
      port "db" {
        to = 6379
      }
    }

    task "redis" {
      driver = "docker"
      config {
        image = "redis:7"
        ports = ["db"]
      }
      resources {
        cpu    = 500
        memory = 256
      }

      env {
        # this will be available as the MOUNT_PATH environment
        # variable in the task
        MOUNT_PATH = "${NOMAD_ALLOC_DIR}/${var.path}"
      }

      volume_mount {
        volume      = "cache-volume"
        destination = "${NOMAD_ALLOC_DIR}/${var.path}"
      }

    }
  }
}
```

The job that results from this job specification has two task groups, each one
named for each of the two volumes. Each allocation has its own volume.

```shell-session
$ nomad job status example
ID            = example
...

Allocations
ID        Node ID   Task Group  Version  Desired  Status   Created  Modified
81d32909  352c6926  cache-1     0        run      running  4s ago   3s ago
ce6fbfc8  352c6926  cache-0     0        run      running  4s ago   3s ago

$ nomad volume status 'test-volume[0]'
...
Allocations
ID        Node ID   Task Group  Version  Desired  Status   Created  Modified
ce6fbfc8  352c6926  cache-0     0        run      running  29s ago  18s ago

$ nomad volume status 'test-volume[1]'
...
Allocations
ID        Node ID   Task Group  Version  Desired  Status   Created  Modified
81d32909  352c6926  cache-0     0        run      running  29s ago  18s ago
```

[volume_mount]: /nomad/docs/job-specification/volume_mount 'Nomad volume_mount Job Specification'
[host_volume]: /nomad/docs/configuration/client#host_volume-block
[csi_volume]: /nomad/docs/commands/volume/register
[csi_plugin]: /nomad/docs/job-specification/csi_plugin
[csi_volume]: /nomad/docs/commands/volume/register
[attachment mode]: /nomad/docs/commands/volume/register#attachment_mode
[volume registration]: /nomad/docs/commands/volume/register#mount_options
