[cinder][nova] Local storage in compute node

Eric K. Miller emiller at genesishosting.com
Thu Aug 6 04:53:49 UTC 2020

> From: Sean Mooney [mailto:smooney at redhat.com]
> Sent: Wednesday, August 05, 2020 8:01 AM
> yes that works well with the default flat/qcow file format
> i assume there was a reason this was not the starting point.
> the nova lvm backend i think does not supprot thin provisioning
> so fi you did the same thing creating the volume group on the nvme deivce
> you would technically get better write performance after the vm is booted
> but
> the vm spwan is slower since we cant take advantage of thin providioning
> and
> each root disk need to be copided  form the cahced image.

I wasn't aware that the nova LVM backend ([libvirt]/images_type = lvm) didn't support thin provisioned LV's.  However, I do see that the "sparse_logical_volumes" parameter indicates it has been deprecated:

That would definitely be a downer.

> so just monting the nova data directory on an nvme driver or a raid of nvme
> drives
> works well and is simple to do.

Maybe we should consider doing this instead.  I'll test with the Nova LVM backend first.

> so there are trade off with both appoches.
> generally i recommend using local sotrage e.g. the vm root disk or ephemeral
> disk for fast scratchpad space
> to work on data bug persitie all relevent data permently via cinder volumes.
> that requires you to understand which block
> devices a local and which are remote but it give you the best of both worlds.

Our use case simply requires high-speed non-redundant storage for self-replicating applications like Couchbase, Cassandra, MongoDB, etc. or very inexpensive VMs that are backed-up often and can withstand the downtime when restoring from backup.

That will be one more requirement (or rather a very nice to have), is to be able to create images (backups) of the local storage onto object storage, so hopefully "openstack server backup create" works like it does with rbd-backed Nova-managed persistent storage.

I will let you know what I find out!

Thanks everyone!


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