Re: [libvirt-users] Blockpull behavior when interrupted

From: "Eric Blake" <eblake(a)redhat.com&gt; To: "Andrew Martin" <amartin(a)xes-inc.com&gt; Cc: libvirt-users(a)redhat.com Sent: Friday, July 22, 2016 10:40:54 PM Subject: 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)