Add options for --disk-only snapshot

[shodanshok]

Continuation of #1639 as requested by @mvollmer

--disk-only snapshot can be useful to take crash-consistent backup of large memory virtual machines with no downtime. Without this option the virtual machine is suspended while its memory is dumped, which can take minutes on large-mem guests and so a significant downtime can be noticed. The fact that --disk-only only creates a crash-consistent snapshot, rather than a fully consistent one, should be clearly communicated to the user.

The ability to delete such a snapshot is critical, while reverting (with the added warning about losing the current memory content) would be nice. Reverting a --disk-only snapshot was not supported by libvirt until recently, but it should be fixed now.

[mvollmer]

Thanks!

I have reopened our old ticket for this, and it's now in the backlog.

--disk-only snapshot can be useful to take crash-consistent backup of large memory virtual machines with no downtime

Thanks especially for this, we were missing a clear ask for the "backup" use case. (I assume "backup" means making an independent, full, off-site copy of the disks of a VM, not merely storing a snapshot delta right next to the disk images.)

Are you talking about the "pull" method mentioned but not explained here, or something else? Is there documentation about the "pull" method somewhere?

What would the next steps be in your backup process after taking the external, disk-only snapshot? Are they similar to "Full backup with older libvirt versions"?

My impression from that section and from knowing how external snapshots are implemented, is that it is quite tricky to actually find the bytes of the snapshot, and Cockpit should probably help with that as well. What do you think?

[shodanshok]

Thanks especially for this, we were missing a clear ask for the "backup" use case. (I assume "backup" means making an independent, full, off-site copy of the disks of a VM, not merely storing a snapshot delta right next to the disk images.)

An off-site copy is a possibility, sure. But even using the snapshot as a point-in-time recovery option would benefit from --disk-only, ie: before doing something inside the guest, take a snapshot without pausing the VM for memory dump, avoiding any service disruption. Then, if something goes wrong, one can stop the guest and restore as appropriate. Doing a full memory dump means that a snapshot can not be easily taken on a busy guests, as it will pause for the duration of the memory save.

[mvollmer]

An off-site copy is a possibility, sure. But even using the snapshot as a point-in-time recovery option would benefit from --disk-only,

Well, now we are back at "I want to have a checkbox for every option of virsh create-snapshot-as". :-)

If we really can't identify any higher level use cases that these options serve, then we have to give up and expose all those options "raw", behind an "Advanced options" expander with a hard hat sign.

I am more open to that idea than others, I guess, and I would like to see Cockpit make more of an effort to demarcate "safe and friendly" things from "pointy and sharp" things, just so that we can offer more pointy and sharp things without people getting hurt all the time by accident.

[mvollmer]

The "hazard area" pattern could look like this:

[shodanshok]

Well, now we are back at "I want to have a checkbox for every option of virsh create-snapshot-as". :-)

A checkbox with something as "disk only snapshot" or "do not include guest memory", with a relevant tooltip/warning stating that a crash-consistent only snapshot is going to be taken, should be enough.

If we really can't identify any higher level use cases that these options serve, then we have to give up and expose all those options "raw", behind an "Advanced options" expander with a hard hat sign.

A disk-only snapshot should be common operation, so I am not sure it deserves to be hidden away, while other advanced options can be under a specific "advanced" tab/label. But this is just my opinion, feel free to ignore it.

I am more open to that idea than others, I guess, and I would like to see Cockpit make more of an effort to demarcate "safe and friendly" things from "pointy and sharp" things, just so that we can offer more pointy and sharp things without people getting hurt all the time by accident.

Thanks for these efforts, I fully agree that potentially dangerous options should be clearly marked as such.