[libvirt-users] Create qcow2 v3 volumes via libvirt

[Daniel P. Berrangé]

On Mon, Apr 30, 2018 at 08:42:56PM +0200, Gionatan Danti wrote:
> Another question: how reliable are qcow2 ver2/3 files nowadays? Are you
> using them in production environments?

qcow2 is widely used in production at large scale in general. Just not
with internal snapshots - almost everything uses external snapshots,
aka backing file chains.

> At the moment, I am using RAW files and filesystem-level snapshot to manage
> versioning; however, as virt-manager has direct support for managing qcow2
> internal snapshots, it would be easier to deploy qcow2 disks.
> 
> What strikes me is that, if thing have not changed, Red Hat support policy
> was to *not* support internal snapshots. So, are they reliable enough for
> production VMs?

The QEMU community still tends to discourage use of internal snapshots.
There are not even any QMP monitor commands to use them - you are forced
to use the legacy HMP interface to QEMU for mgmt. All of the workaround
providing interesting block storage mgmt is focused on external snapshots
(aka the backing_file option). There are some technical downsides to
internal snapshots IIUC, such as inability to free the space used by the
internal snapshot when it is deleted, loading/saving snapshots blocks
execution of the guest OS, and probably more I've forgotten about.

The only nice thing about internal snapshots is simplicity of mgmt, and
that is a very nice thing indeed, which is why virt-manager has code
to support that - it was much easier to add that code for external
snapshots. Just a shame about all the downsides :-(

Regards,
Daniel
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