[pve-devel] Proposal: support for atomic snapshot of all VM disks at once

[Max Carrara]

On Wed Nov 20, 2024 at 5:10 PM CET, Ivaylo Markov wrote:
> Hello,
>
> I've been caught up in other things and it's been a while, but as I was 
> collating and testing my proposed changes, I came across this again and 
> thought I'd clarify something.
>
> On 08/10/2024 13:50, Max Carrara wrote:
> >> I was directed here to discuss this proposal and my implementation idea
> >> after an initial post in Bugzilla[1]. The goal is to give storage
> >> plugins the option to perform atomic crash-consistent snapshots of the
> >> virtual disks associated with a virtual machine where the backend
> >> supports it  (e.g. Ceph, StorPool, and ZFS) without affecting those
> >> without such a feature.
> > Since you mentioned that the backends without such a feature won't be
> > affected, will the disks of the storage types that *do* support it still
> > be addressable individually? As in: Would it be possible to run both
> > group snapshots and individual snapshots on a VM's disks? (I'm assuming
> > that they will, but still wanted to ask.)
> >
> Do you mean this this as "you can snapshot the whole VM (all disks) at 
> once *or* each disk individually" or "when making a snapshot of the 
> entire VM, the user can chose between individual/group snapshot of all 
> drives". What I have so far matches the first description, but I just 
> realized you might have meant the second one, so I thought I'd ask and 
> potentially create more work for myself :)

Hey!

No worries, always feel free to ask :)

I should have phrased myself better there, so I'll try to be as specific
as possible here. Let me define some terms first:

- regular snapshots: This is how things work currently; a snapshot of
  each disk of the VM is made after the previous one finishes. In other
  words, disk snapshots are made *sequentially*. Works for every storage
  type.

- group snapshots: What you're implementing -- essentially creating a
  snapshot of *all* disks of a VM at once, atomically. Snapshots of this
  type are crash-consistent. Only works for storage types that support
  it.

What I meant to ask was whether *both* options will remain available to
users. For example, if I click on a VM, then go to Snapshots -> Take
Snapshot, will I have the option to say whether the snapshot should be a
group snapshot or a regular snapshot?

I guess calling it "individual" was a bit ambiguous here; I didn't mean
that I could e.g. snapshot just one disk only (if that's what confused
you). When performing a snapshot of a VM, a snapshot for *all* disks of
the VM is *always* made, regardless of whether a regular snapshot or a
group snapshot is performed on a VM.

I hope all that isn't too long -- just wanted to make sure everything's
clear! :)