Reliably share a persistent filesystem between units

[Robin Winslow]

Does anyone know of the best way to share a folder between Juju units in a
persistent and reliable way?

Up until now we've been dealing with shared data that needs persistence by
storing it in Swift, and so I have written my applications to interact with
a Swift server directly (and therefore the need to be provided with the
Swift credentials).

However, I now have a situation where it would be much neater if the
application could simply interact with a local folder without any knowledge
of the underlying storage system. So I need a way to have the data in that
folder reliably shared between all the application units with Juju, and
also persisted somewhere outside the deployment in case the environment is
destroyed.

I assume the sharing could be simply achieved using NFS
<https://jujucharms.com/nfs/> or similar, but AFAIK that doesn't in and of
itself provide any redundancy or help with persisting the data.

Has anyone done anything like this?

>From looking into it, it looks like CephFS
<http://docs.ceph.com/docs/master/cephfs/> might do what I want (and it
looks like
<http://docs.ceph.com/docs/master/cephfs/best-practices/#which-ceph-version>
it became officially "stable" in the Jewel release
<http://ceph.com/releases/v10-2-2-jewel-released/> on June 15th), allowing
me to mount a remote Ceph setup at a specific folder within my unit
(correct me if I'm wrong here). However I have no prior experience with
setting up or interacting with Ceph either within or outside Juju.

Has anyone implemented CephFS before? Does anyone know if it can be done
with existing Juju charms? If not, I'd be happy to try writing something,
although Ceph will be a significant learning curve for me. It looks like it
needs a Ceph Metadata Server (MDS) which I couldn't see a charm for
<https://jujucharms.com/q/ceph>.

Or is there a simpler solution that makes sense? Perhaps I'm overestimating
the problems with using the NFS charm for this?

Any help would be much appreciated. Thanks.
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