Disk hotplug and snapshots

[Federico Simoncelli]

----- Original Message -----
> From: "Mike Kolesnik" <mkolesni at redhat.com>
> To: "Federico Simoncelli" <fsimonce at redhat.com>
> Cc: "oVirt Arch" <arch at ovirt.org>
> Sent: Tuesday, March 6, 2012 10:47:57 AM
> Subject: Re: Disk hotplug and snapshots
> 
> > Hi,
> >  looking at the disk hotplug feature in the wiki I was wondering
> >  how
> > that fits with the current implementation of the snapshots on the
> > engine
> > side.
> > As far as I know the snapshots are at VM level now (no sigle disk
> > snapshots),
> > so what happens when you hotplug a new disk (no snapshots) into a
> > VM
> > that
> > already has one or more snapshots? (and the other way around if
> > possible).
> 
> What do you mean by 'hotplug'?
> By feature description, the disk is always part of VM even if it's
> unplugged.
> This is not the same as adding/removing disk to/from VM..

The real value of this feature is being able to dynamically add and
remove disks from a running VM.
Even if technically there's a difference (and btw only in the specific
implementation in the engine) between hotplugging and adding/removing
disks to/from a VM, they'll often be used together.

> Snapshots should contain configuration which states if disk is
> plugged/unplugged - so if this configuration is present, it is used
> to determine the state of disks at that snapshot.
> 
> Snapshots also contain a list of the disks that existed when snapshot
> was taken, so adding a new disk will not affect old snapshots either
> way.
> You can then hotplug this disk, but this has no affect on existing
> snapshots.
> 
> When you unplug disk, it is still part of the VM and this also
> doesn't affect past snapshots.

One of the uses of the disk hotplug is being able to move disks from
one VM to another. We have two special cases here:

 1. you're removing a disk that was part of previous snapshot on the
    old VM (what happens if the user tries to revert? any spurious
    error since the disk is not there anymore?)
 2. you're adding the disk that already contains a snapshot to a new
    VM, and you won't be able to control it anymore (merge, revert).

Is this scenario not supported yet?

-- 
Federico