[Engine-devel] Question about CloneVMFromSnapshot feature in context of shared disks and direct LUNs-based disks

[Ayal Baron]

----- Original Message -----
> On 01/19/2012 10:32 AM, Mike Kolesnik wrote:
> > 
> > 
> > ----- Original Message -----
> >>
> >>
> >> ----- Original Message -----
> >>> From: "Livnat Peer" <lpeer at redhat.com>
> >>> To: "Yair Zaslavsky" <yzaslavs at redhat.com>, "Mike Kolesnik"
> >>> <mkolesni at redhat.com>
> >>> Cc: engine-devel at ovirt.org
> >>> Sent: Thursday, January 19, 2012 9:19:52 AM
> >>> Subject: Re: [Engine-devel] Question about CloneVMFromSnapshot
> >>> feature in context of shared disks and direct
> >>> LUNs-based disks
> >>>
> >>> On 19/01/12 08:38, Yair Zaslavsky wrote:
> >>>> Hi all,
> >>>> Following the upstream meeting dated Wednesday January 18th,
> >>>> 2012
> >>>> -
> >>>> I presented the clone VM from snpashot feature and we discussed
> >>>> the
> >>>> feature behaviour.
> >>>>
> >>>> Two issues that were raised are the behaviour of the feature in
> >>>> context
> >>>> of shared disks and direct LUNs-based disks -
> >>>> On one hand, if we copy&collapse such images - this may yield to
> >>>> data
> >>>> corruption (think of a scenario where the source and destination
> >>>> VMs use
> >>>> the same disk).
> >>>> On the other hand - if we decide not to copy&collapse  - the
> >>>> target
> >>>> VM
> >>>> will have missing VM and its state will not totally reflect the
> >>>> logical
> >>>> state.
> >>>> One of the solution raises is to mark such disks (at the
> >>>> destination) as
> >>>> unplugged, allowing the administrator the ability to plug them
> >>>> (understanding of course the consequences).
> >>>>
> >>>> I would like to receive inputs on this issue
> >>>>
> >>>>
> >>>> Kind regards,
> >>>>
> >>>> Yair
> >>>
> >>> Hi Yair,
> >>>
> >>> Some clarifications on the above issue.
> >>> Currently when taking a snapshot on a VM with shared disks or
> >>> direct
> >>> LUN
> >>> disk there are 3 optional behaviors:
> >>>
> >>> 1. Blocking the snapshot action. (User can not take a snapshot of
> >>> the
> >>> VM
> >>> if it has plugged shared or direct LUN disks)
> >>>
> >>> 2. Taking the snapshot and marking the shared disk and direct LUN
> >>> disks
> >>> as unplugged (in the VM snapshot configuration) and marking the
> >>> snapshot
> >>> state as partial.
> >>>
> >>> 3. Taking the snapshot of the VM as is, leaving the VM
> >>> configuration
> >>> with plugged disks.
> >>>
> >>>
> >>> The issue with including these disks in the snapshot is that they
> >>> are
> >>> not really being snapshotted, they are not capturing the point in
> >>> time
> >>> we are trying to achieve in the snapshot.
> >>>
> >>> Enabling the snapshot action in such a state is a bit misleading
> >>> to
> >>> the
> >>> user.
> >>>
> >>> If we do allow taking the snapshot we should mark the snapshot as
> >>> partial to indicate that the snapshot did not capture the point
> >>> in
> >>> time
> >>> as the user intended.
> >>>
> >>> I have no preference with regards to the second and third
> >>> approach,
> >>> the
> >>> second approach is a bit more safe, we basically force the user
> >>> to
> >>> plug
> >>> the disks and be sure that he knows what he is doing and the
> >>> third
> >>> approach is less safe and less annoying to the user (he took the
> >>> snapshot, cloned it and wants to start the VM - don't require
> >>> extra
> >>> actions)
> >>>
> >>> Kolesnik - please note when starting VM in a preview mode we
> >>> should
> >>> mount the disks in read-only mode (if supported).
> > 
> > I don't understand this, can you please elaborate why and in which
> > case?
> >   The disk is plugged/unplugged?
> >   What happens when you commit? It becomes r/w?
> > 
> >>>
> >>>
> >>> Livnat
> >>>
> >>>
> >>>
> >>
> >> +1 for option 3
> >>
> > 
> > +1 for option 3 as well (also good with option 1, but I think this
> > will hinder usability).
> I agree with Mike - I think option one is too "aggressive" in
> protecting
> us, this is why I prefer 3.  +1 on option 3

This would only be acceptable if the disk is marked read only.
Just leaving the disk plugged means many users *will* corrupt their VMs.
That trumps the need to mark a checkbox if you want it available.

As I said before, readonly has its own problems and in fact, IMO the behaviour is even more difficult to explain (yeah, you might corrupt the running VM if it's r/w so we made it r/o and now if you start up your clone / preview you will get a kernel panic).

> > 
> > 
> > Regards,
> > Mike
> 
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