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Re: [libvirt-users] Blockpull behavior when interrupted

On 07/22/2016 11:30 AM, Andrew Martin wrote:

> Thanks, that was a helpful overview. To clarify, block commit is changing
> the backing files,but even though it is doing that, the data is consistent
> even if it is interrupted?

Yes, the guest's view of the data is consistent.  Intermediate backing
files may have contents that don't correspond to any historical state
seen by the guest, but the guest view is never compromised.

> Reviewing the presentation brought up another question. I have been thinking
> about installing qemu-guest-agent so I can use --quiese with snapshot-create-as,
> however I have a couple of questions:
> * does the guest agent always report back to libvirt on the host if it succeeds,
> fails, or has an error when attempting to sync(2)?

Guest agents only work if you can trust the guest (a malicious guest
could just lie), but in general, for a trusted guest, yes, the agent
will inform the host if there was an error quiescing data, at which
point libvirt will mark the overall command as failed.

> * isn't it a security risk to allow the guest the ability to communicate to the
> host via the guest agent port?

It's always a security risk to trust a random guest to do anything, if
you don't trust the guest to begin with.  --quiesce, and the guest agent
in general, is only useful for trusted guests.

> Also, any thoughts on utilizing ZFS snapshots in lieu of internal or external
> qcow2 snapshots, at least for backup purposes? Utilizing zfs send/zfs receive
> would be nice for performance when sending full image backups to a remote server.

Patches are welcome, although ZFS may be tricky because of licensing
questions (I'm not a lawyer, but I know enough to not touch ZFS as long
as its current license doesn't play nicely with the Linux kernel)

Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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