Coming in Juju 2.3: storage improvements

I’ve just about wrapped up a set of improvements to storage for Juju 2.3, the next “minor” release. If you’re already using, or have been planning to use Juju’s storage support, read on.

Dynamic storage management

Juju charms can specify storage requirements: the number of filesystems or block devices its application requires. For example, the PostgreSQL charm requires a filesystem on which to store the database. If you don’t tell Juju otherwise, the storage will go onto the root filesystem, but you can also tell Juju to provide the charm with cloud storage (Amazon EBS, OpenStack Cinder, etc.)

One of the missing pieces that users have been asking for is the ability to manage the lifecycle of storage independently of applications and units, and to reuse existing storage. In Juju 2.3, when you remove an application or unit, the storage attached to the unit(s) will (if possible) be detached, rather than destroyed, and will remain in the model. You can then either remove the storage using juju remove-storage, or attach it to a new unit using the new juju attach-storage command, or the --attach-storage flag added to juju deploy and juju add-unit. To complement juju attach-storage, there is also a new juju detach-storage command.

So to illustrate, you can now deploy PostgreSQL with cloud storage, then remove the application, and redeploy (e.g. with more RAM), using the same storage.

juju deploy postgresql --storage=10G
juju remove-application postgresql
juju deploy postgresql --constraints mem=16G --attach-storage pgdata/0

We’re still working on giving you commands to remove storage from the model without destroying it, and then import it into a new model (possibly new controller). This is required for disaster recovery. Whether this makes it for 2.3 depends on prioritisation; if it doesn’t make it for 2.3, it shouldn’t be far behind.

LXD Storage Provider

One thing that we hadn’t planned for 2.3, but we did manage to get done, is a LXD storage provider. LXD has recently added its own storage management API, and Juju 2.3 will have a storage provider that uses it. I originally implemented the Juju side of things as a bit of a hack, behind a feature flag, in order to speed up the development of the aforementioned attach/detach changes. The LXD storage API turned out to be very straight forward to build on, so we decided to release the Juju changes into the wild in case it’s of use to others. Particularly if you’re developing or testing charms that use storage, this should be useful.

Using the LXD storage provider is as simple as:

juju deploy postgresql --storage=10G,lxd

Each storage pool using the “lxd” storage provider will create an associated storage in LXD. When you create a storage pool in Juju, you need to specify two configuration attributes:

  • the LXD storage pool name, as the “lxd-pool” attribute
  • the LXD storage driver, as the “driver” attribute

You can also define driver-specific attributes, which will be passed through to the LXD storage driver verbatim.

Juju predefines a “lxd-zfs” pool, with the following attributes:

  • lxd-pool=juju-zfs
  • driver=zfs
  • zfs.pool_name=juju-zfs

If you deploy an application with storage using the lxd-zfs pool, Juju will create a LXD storage pool called “juju-zfs” with the “zfs” driver, and ZFS pool called “juju-zfs”. To find out more about the LXD storage driver options, see the LXD storage docs.

Posted July 13, 2017. Tags: juju, storage, lxd.