Using Juju Storage

Certain applications can benefit from advanced storage configurations and if a charm exists for such an application Juju can declare such requirements both at deploy time and during the lifetime of the application.

The level of sophistication is limited by the charm; a charm may support multiple storage options (e.g. persistent storage, additional cache). All this allows the user to allocate resources at a granular level. This page will refer to the PostgreSQL charm and the Ceph OSD charm.

The Ceph examples used here are based on the Ceph cluster described in the document Installing Ceph.

Storage management commands

Outside of application deployment, Juju also has a wide array of storage management abilities. Related commands are listed below, along with a brief description of each.

Creates and attaches a storage instance to a unit.
Attaches an existing storage instance to a unit.
Creates or defines a storage pool.
Detaches a storage instance from a unit. Storage is preserved.
Imports a filesystem into a model.
Removes a storage instance from a model. Storage is destroyed.
Shows details of a storage instance.
Lists all storage instances in a model.
Lists all storage pools in a model.

Storage constraints

Several properties are used to dictate how storage is allocated:

  • 'pool': class of storage
  • 'size': size of each volume
  • 'count': number of volumes

The default pool (e.g. 'ebs' for AWS, 'cinder' for OpenStack) is given by:

juju model-config storage-default-block-source

These properties are specified as constraints with the juju deploy or juju add-storage commands.

juju deploy

juju deploy <charm> [--storage <label>=<pool>,<size>,<count>]


  • label is a string taken from the charm itself. It encapsulates a specific storage option/feature. Sometimes called a store.
  • --storage may be specified multiple times, to support multiple labels.

If at least one constraint is specified the following default values come into effect:

  • 'pool' = the default pool (see above)
  • 'size' = determined from the charm's minimum storage size, or 1GiB if the charm does not specify a minimum
  • 'count' = the minimum number required by the charm, or '1' if the storage is optional

In the absence of any storage constraints, the storage will be put on the root filesystem.

juju add-storage

juju add-storage <unit> <label>[=<pool>,<size>,<count>]

As with juju add-unit the storage parameters used are taken from the juju deploy command corresponding to the unit.


To deploy PostgreSQL with one instance (count) of 100GiB, via the charm's 'pgdata' storage label, using the default storage pool:

juju deploy postgresql --storage pgdata=100G

Assuming an AWS model, a more explicit, but equivalent, command is:

juju deploy postgresql --storage pgdata=ebs,100G,1

Storage pools

Use the juju storage-pools command to list the predefined storage pools as well as any custom ones that may have been created with the juju create-storage-pool command:

juju storage-pools

Here is sample output for a newly-added AWS model:

Name     Provider  Attrs
ebs      ebs       
ebs-ssd  ebs       volume-type=ssd
loop     loop      
rootfs   rootfs    
tmpfs    tmpfs     

Note: The name given to a default storage pool will often be the same as the name of the storage provider upon which it is based.

Depending on the storage provider (see below), custom storage pools can be created. In the case of AWS, the 'ebs' storage provider supports several configuration attributes:

  • 'volume-type': volume type (i.e. magnetic, ssd, or provisioned-iops)
  • 'encrypted': enable/disable disk encryption
  • 'iops': IOPS per GiB

For example, to provision a 3000 IOPS volume (100GiB x 30IOPS/GiB) by first creating a custom storage pool and then having a newly deployed PostgreSQL use it for its database storage:

juju create-storage-pool iops ebs volume-type=provisioned-iops iops=30
juju deploy postgresql --storage pgdata=iops,100G

See IOPS (Wikipedia) for background information.

Dynamic storage

Most storage can be dynamically added to, and removed from, a unit. Some types of storage, however, cannot be dynamically managed. For instance, Juju cannot disassociate MAAS disks from their respective MAAS nodes. These types of static storage can only be requested at deployment time and will be removed when the machine is removed from the model.

Certain cloud providers may also impose restrictions when attaching storage. For example, attaching an EBS volume to an EC2 instance requires that they both reside within the same availability zone. If this is not the case, Juju will return an error.

When deploying an application or unit that requires storage, using machine placement (i.e. --to) requires that the assigned storage be dynamic. Juju will return an error if you try to deploy a unit to an existing machine, while also attempting to allocate static storage.

Adding and detaching storage

Assuming the storage provider supports it, storage can be created and attached to a unit using juju add-storage. Juju will ensure the storage is allowed to attach to the unit's machine.

Charms can specify a maximum number of storage instances. In the case of the charm 'postgresql', a maximum of one is allowed for 'pgdata'. If an attempt is made to exceed it, Juju will return an error.

Dynamic storage can be detached from units using juju detach-storage.

Charms can also define a minimum number of storage instances. The postgresql charm specifies a minimum of zero for 'pgdata' whereas another charm may specify a different number. In any case, if detaching storage from a unit would bring the total number of storage instances below the minimum, Juju will return an error.

It is not possible to add new storage to a model without also attaching it to a unit. However, with the juju import-filesystem command, you can add storage to a model that has been previously released from a removed model.


To create a 32GiB EBS volume and attach it to unit 'ceph-osd/0' as its OSD storage:

juju add-storage ceph-osd/0 osd-devices=ebs,32G,1

Above, the volume was created in the same availability zone as the instance (a requirement).

To detach OSD device 'osd-devices/2' from a Ceph unit:

juju detach-storage osd-devices/2

Important: Detaching storage from a unit does not destroy the storage.


As we saw, detaching storage does not destroy the storage. In addition, when a unit is removed from a model, and the unit has dynamic storage attached, the storage will be detached and left intact. This allows detached storage to be re-attached to an existing unit using juju attach-storage, or to a new unit using the --attach-storage flag of juju deploy or juju add-unit.

The underlying cloud's storage resource is normally destroyed by first detaching it and then using juju remove-storage. To remove storage from the model without destroying it the --no-destroy option must be used. Be wary of using the latter option as Juju will lose sight of the volume; it will only be visible from the cloud provider.

If an attempt is made to either attach or remove storage that is currently in use (i.e. it is attached to a unit) Juju will return an error. To remove currently attached storage from the model the --force option must be used.

Finally, a model cannot be destroyed while storage volumes remain without passing a special option (--release-storage to detach all volumes and --destroy-storage to remove all volumes). Naturally, this applies to the removal of a controller as well.


To attach existing storage 'osd-devices/7' to existing unit 'ceph-osd/1':

juju attach-storage ceph-osd/1 osd-devices/7

To deploy PostgreSQL with (detached) existing storage 'pgdata/0':

juju deploy postgresql --attach-storage pgdata/0

Note: The --attach-storage and -n flags cannot be used together.

To add a new Ceph OSD unit with (detached) existing storage 'osd-devices/2':

juju add-unit ceph-osd --attach-storage osd-devices/2

To remove already detached storage 'osd-devices/3' from the model. It will also be automatically destroyed on the cloud provider:

juju remove-storage osd-devices/3

To remove currently attached storage 'pgdata/1' from the model and prevent it from being destroyed on the cloud provider:

juju remove-storage --force --no-destroy pgdata/1

To upgrade the OSD journal of Ceph unit 'ceph-osd/0' from magnetic to solid state (SSD) and dispose of the unneeded original journal 'osd-journals/0':

juju add-storage ceph-osd/0 osd-journals=ebs-ssd,8G,1
juju detach-storage osd-journals/0
juju remove-storage osd-journals/0

To destroy a controller (and its models) along with all existing storage volumes:

juju destroy-controller lxd-controller --destroy-all-models --destroy-storage

To destroy a model while keeping intact all existing storage volumes:

juju destroy-model default --release-storage

Assuming the above model was LXD-based, to create a new model and import the released storage volume into it, giving it a storage name of 'pgdata':

juju add-model default
juju import-filesystem lxd juju:juju-7a544c-filesystem-0 pgdata

The determination of the provider ID (juju:juju-7a544c-filesystem-0) is dependent upon cloud type. Above, it is given by the backing LXD pool and the volume name (obtained with lxc storage volume list <lxd-pool>), all separated by a :. A provider ID from another cloud may look entirely different. The LXD storage provider and associated LXD pools are described in detail below.

Cross-model storage

Storage management is currently restricted to a single model, which means it is not possible to reuse storage from one model/controller in another. Also, when a model/controller is removed, all associated storage will be destroyed. Support for releasing storage from a model, and enlisting it into another, is planned for a future release.

Upgrading charms

When upgrading a charm with the juju upgrade-charm command, the existing storage constraints specified at deployment time will be preserved.

It is also possible to change the storage constraints and define new ones by passing the --storage flag to juju upgrade-charm. For example, if the 'pgdata' storage option did not exist in revision 1 of the postgresql charm, but was introduced in revision 2, when upgrading (from 1 to 2) you could do:

juju upgrade-charm postgresql --storage pgdata=10G

If such a constraint was not provided, 'rootfs' would be used (as described in the section on deploying with storage constraints).

Warning: Specifying new constraints may be necessary when upgrading to a revision of a charm that introduces new, required, storage options.

Storage providers

Generic storage providers

There are several cloud-independent storage providers, which are available to all types of models:

  • loop

    Block-type, creates a file on the unit's root filesystem, associates a loop device with it. The loop device is provided to the charm.

  • rootfs

    Filesystem-type, creates a sub-directory on the unit's root filesystem for the unit/charm to use.

  • tmpfs

    Filesystem-type, creates a temporary file storage facility that appears as a mounted file system but is stored in volatile memory.

Loop devices require extra configuration to be used within LXD. For that, please refer to Loop devices and LXD (below).

AWS/EBS (ebs)

AWS-based models have access to the 'ebs' storage provider, which supports the following pool attributes:

  • volume-type

    Specifies the EBS volume type to create. You can use either the EBS volume type names, or synonyms defined by Juju (in parentheses):

    • standard (magnetic)
    • gp2 (ssd)
    • io1 (provisioned-iops)
    • st1 (optimized-hdd)
    • sc1 (cold-storage)

    Juju's default pool (also called 'ebs') uses gp2/ssd as its own default.

  • iops

    The number of IOPS for provisioned-iops volume types. There are restrictions on minimum and maximum IOPS, as a ratio of the size of volumes. See Provisioned IOPS (SSD) Volumes for more information.

  • encrypted

    Boolean (true|false); indicates whether created volumes are encrypted.

For detailed information regarding EBS volume types, see the AWS EBS documentation.

OpenStack/Cinder (cinder)

OpenStack-based models have access to the 'cinder' storage provider.

The 'cinder' storage provider has a 'volume-type' configuration option whose value is the name of any volume type registered with Cinder.

MAAS (maas)

MAAS has support for discovering information about machine disks, and an API for acquiring nodes with specified disk parameters. Juju's MAAS provider has an integrated 'maas' storage provider. This storage provider is static-only; it is only possible to deploy charms using 'maas' storage to a new machine in MAAS, and not to an existing machine, as described in the section on dynamic storage.

The MAAS provider currently has a single configuration attribute:

  • tags

    A comma-separated list of tags to match on the disks in MAAS. For example, you might tag some disks as 'fast'; you can then create a storage pool in Juju that will draw from the disks with those tags.

Microsoft Azure (azure)

Azure-based models have access to the 'azure' storage provider.

The 'azure' storage provider has an 'account-type' configuration option that accepts one of two values: 'Standard_LRS' and 'Premium_LRS'. These are, respectively, associated with defined Juju pools 'azure' and 'azure-premium'.

Newly-created models configured in this way use "Azure Managed Disks". See Azure Managed Disks Overview for information on what this entails (in particular, what the difference is between standard and premium disk types).

Google Compute Engine (gce)

Google-based models have access to the 'gce' storage provider. The GCE provider does not currently have any specific configuration options.

Oracle Compute Cloud (oracle)

Oracle-based models have access to the 'oracle' storage provider. The Oracle provider currently supports a single pool configuration attribute:

  • volume-type

    Volume type, a value of 'default' or 'latency'. Use 'latency' for low-latency, high IOPS requirements, and 'default' otherwise.

    For convenience, the Oracle provider registers two predefined pools:

    • 'oracle' (volume type is 'default')
    • 'oracle-latency' (volume type is 'latency').

LXD (lxd)

Note: The regular package archives for Ubuntu 14.04 LTS (Trusty) and Ubuntu 16.04 LTS (Xenial) do not include a version of LXD that has the 'lxd' storage provider feature. You will need at least version 2.16. See the Using LXD with Juju page for installation help.

LXD-based models have access to the 'lxd' storage provider. The LXD provider has two configuration options:

  • driver

    This is the LXD storage driver (e.g. zfs, btrfs, lvm, ceph).

  • lxd-pool

    The name to give to the corresponding storage pool in LXD.

Any other parameters will be passed to LXD (e.g. zfs.pool_name). See upstream LXD storage configuration for LXD storage parameters.

Every LXD-based model comes with a minimum of one LXD-specific Juju storage pool called 'lxd'. If ZFS and/or BTRFS are present when the controller is created then pools 'lxd-zfs' and/or 'lxd-btrfs' will also be available. The following output to the juju storage-pools command shows all three Juju LXD-specific pools:

Name       Provider  Attrs
loop       loop
lxd        lxd
lxd-btrfs  lxd       driver=btrfs lxd-pool=juju-btrfs
lxd-zfs    lxd       driver=zfs lxd-pool=juju-zfs zfs.pool_name=juju-lxd
rootfs     rootfs
tmpfs      tmpfs

As can be inferred from the above output, for each Juju storage pool based on the 'lxd' storage provider there is a LXD storage pool that gets created. It is these LXD pools that will house the actual volumes.

The LXD pool corresponding to the Juju 'lxd' pool doesn't get created until the latter is used for the first time (typically via the juju deploy command). It is called simply 'juju'.

The command lxc storage list is used to list LXD storage pools. A full "contingent" of LXD non-custom storage pools would like like this:

|    NAME    | DESCRIPTION | DRIVER |               SOURCE               | USED BY |
| default    |             | dir    | /var/lib/lxd/storage-pools/default | 1       |
| juju       |             | dir    | /var/lib/lxd/storage-pools/juju    | 0       |
| juju-btrfs |             | btrfs  | /var/lib/lxd/disks/juju-btrfs.img  | 0       |
| juju-zfs   |             | zfs    | /var/lib/lxd/disks/juju-zfs.img    | 0       |

To be clear, the three Juju-related pools above are for storing volumes that Juju applications can use. The fourth 'default' pool is the standard LXD storage pool where the actual containers (operating systems) live.

To deploy an application, refer to the pool as usual. Here we deploy PostgreSQL using the 'lxd' Juju storage pool, which, in turn, uses the 'juju' LXD storage pool:

juju deploy postgresql --storage pgdata=lxd,8G

See Using LXD with Juju for how to use LXD in conjunction with Juju, including the use of ZFS as an alternative filesystem.

Loop devices and LXD

LXD (localhost) does not officially support attaching loopback devices for storage out of the box. However, with some configuration you can make this work.

Each container uses the 'default' LXD profile, but also uses a model-specific profile with the name juju-<model-name>. Editing a profile will affect all of the containers using it, so you can add loop devices to all LXD containers by editing the 'default' profile, or you can scope it to a model.

To add loop devices to your container, add entries to the 'default', or model-specific, profile, with lxc profile edit <profile>:

    major: "10"
    minor: "237"
    path: /dev/loop-control
    type: unix-char
    major: "7"
    minor: "0"
    path: /dev/loop0
    type: unix-block
    major: "7"
    minor: "1"
    path: /dev/loop1
    type: unix-block
    major: "7"
    minor: "9"
    path: /dev/loop9
    type: unix-block

Doing so will expose the loop devices so the container can acquire them via the losetup command. However, it is not sufficient to enable the container to mount filesystems onto the loop devices. One way to achieve that is to make the container "privileged" by adding:

  security.privileged: "true"

Writing charms

For guidance on how to create a charm that uses these storage features see Writing charms that use storage.

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