Many applications require access to a storage resource of some form. Juju charms can declare what storage requirements they have, and these can be allocated when the charm is deployed. Charms may declare several types of storage requirement (e.g. for persistent storage and an additional cache) so that resources can be allocated at a more granular level.
Juju has the
juju storage command and
subcommands to create and manage storage resources. All commands and
subcommands accept the “--help” flag for usage and help information.
juju storage --help juju add-storage juju show-storage juju create-storage-pool juju storage-pools
For this document, we will use a charm which has been modified to support storage: https://code.launchpad.net/~axwalk/charms/trusty/postgresql/trunk.
By default, charms with storage requirements will allocate those resources on
the root filesystem of the unit where they are deployed. To make use of
additional storage resources, Juju needs to know what they are. Some providers
(e.g. EC2) support generic default storage pools (see the documentation on
provider support), but in the case of no default support or
a desire to be more specific, use the
juju storage pool create subcommand to
juju create-storage-pool loopy loop size=100M juju create-storage-pool rooty rootfs size=100M juju create-storage-pool tempy tmpfs size=100M
juju storage-pools loopy: provider: loop attrs: size: 100M rooty: provider: rootfs attrs: size: 100M tempy: provider: tmpfs attrs: size: 100M
If the storage provider supports dynamically adding storage to a machine, then an application/unit deployed with storage may be placed on an existing machine. Not all providers support dynamic storage; for example, MAAS provides an interface to physical hardware.
All environment providers support the following storage providers:
block-type, creates a file in the agent data-dir and attaches a loop device to it. See the Known limitations section below for a comment on using the loop storage with the local/LXC provider.
filesystem-type, creates a sub-directory in the agent's data-dir for the unit/charm to use.
filesystem-type, creates a temporary file storage facility that appears as a mounted file system but is stored in volatile memory.
Additionally, native storage providers exist for the several major cloud providers, described below.
The EC2/EBS provider currently supports the following pool configuration attributes:
specifies the EBS volume type to create. You can use either the EBS volume type names, or synonyms defined by Juju (in parentheses): gp2 (ssd), io1 (provisioned-iops), standard (magnetic). By default, magnetic/standard volumes will be created. An 'ebs-ssd' pool is created in all EC2 environments, which defaults the volume type to ssd/gp2 instead.
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.
true|false, indicating whether or not to encrypt volumes created by the pool.
For information regarding EBS volume types, see the EBS documentation.
The OpenStack/Cinder provider does not currently have any specific configuration options.
OpenStack defaults to using Cinder for additional specified storage, so it is possible to use cinder storage like this:
juju deploy postgresql --storage pgdata=10G
which will create a 10G Cinder volume. Or if you wish to be more specific:
juju deploy postgresql --storage pgdata=cinder,10G
will achieve the same result.
MAAS 1.8+ contains support for discovering information about machines' 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 currently only possible to deploy charms requiring block storage to a new machine in MAAS, and not to an existing machine.
The MAAS provider currently has a single configuration attribute:
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.
The Microsoft Azure provider does not currently have any storage configuration.
The Google Compute Engine provider does not currently have any storage configuration.
A charm which requires storage will have the default storage (unit filesystem) allocated for it automatically. Constraints can be used, when deploying an application, to override the default requirements.
The constraints can specify the type/pool, size and count, of the storage required. At least one of the constraints must be specified, but otherwise they are all optional.
If pool is not specified, then Juju will select the default storage provider for the current environment (e.g. cinder for openstack, ebs for ec2, loop for local). If size is not specified, then Juju will use the minimum size specified in the charm's storage metadata, or 1GiB if the metadata does not specify. If count is not specified, then Juju will create a single instance of the store.
juju deploy <charm> --storage <label>=<pool>,<size>,count
For example, to deploy the postgresql service and have it use the unit’s local filesystem for 10 gibibytes of storage for its ‘data’ storage requirement:
juju deploy postgresql --storage pgdata=rootfs,10G
We can also deploy using a local loop device
juju deploy postgresql --storage pgdata=loop,5G
If the size is omitted...
juju deploy postgresql --storage pgdata=rootfs
Juju will use a default size of 1GiB, unless the charm itself has specified a minimum value, in which case that will be used.
When deploying on a provider which supplies storage, the supported storage pool types may be used in addition to ‘loop’ and ‘rootfs’. For example, on using Amazon’s EC2 provider, we can make use of the default ‘ebs’ storage pool
juju deploy postgresql --storage pgdata=ebs,10G
Cloud providers may support more than one type of storage. For example, in the case of EC2, we can also make use of the ebd-ssd pool, which is SSD-based storage, and hence faster and better for some storage requirements.
juju deploy postgresql --storage pgdata=ebs-ssd
We can also merely specify the size, in which case Juju will use the default pool for the selected environment. E.g.:
juju deploy postgresql --storage pgdata=10G
Which, on the EC2 provider, will create a 10 gibibyte volume in the ‘ebs’ pool.
Charms may declare multiple types of storage, in which case they may all be specified using the constraint, or some or all can be omitted to accept the default values:
juju deploy postgresql --storage pgdata=ebs,10G cache=ebs-ssd
When updating a charm with the upgrade-charm command, default storage constraints will be preserved unless new constraints have been added to the updated charm.
For example, if an update to the PostgreSQL charm adds a requirement for pgdata and pgdata doesn't currently exist, the update will automatically create a rootfs pgdata storage instance for each unit.
As with the
deploy command, constraints can be specified when updating
by adding the '--storage' argument:
juju upgrade-charm postgresql --storage pgdata=10G
LXD (localhost) does not officially support mounting loopback devices for storage. However, with some configuration you can make this work.
Each container uses the default profile, but also uses a model-specific profile
with the name juju-
To add loop devices to your container, add loop device entries to the default or model-specific profile, like this:
... devices: loop-control: major: "10" minor: "237" path: /dev/loop-control type: unix-char loop0: major: "7" minor: "0" path: /dev/loop0 type: unix-block loop1: major: "7" minor: "1" path: /dev/loop1 type: unix-block ... loop9: major: "7" minor: "9" path: /dev/loop9 type: unix-block
The above is enough to expose the loop devices into the container, and for the container to acquire one of them using "losetup". It is not yet enough to enable the container to mount filesystems on the loop devices. For that, the simplest thing to do is to make the container privileged by adding:
config: security.privileged: "true"
If you are interested in more information on how to create a charm that uses the storage feature read writing charms that use storage.