[Engine-devel] Adding atomic restore snapshot command at backend

[Omer Frenkel]

----- Original Message -----
> From: "Michael Pasternak" <mpastern at redhat.com>
> To: "Livnat Peer" <lpeer at redhat.com>
> Cc: "engine-devel" <engine-devel at ovirt.org>
> Sent: Monday, May 28, 2012 2:37:10 PM
> Subject: Re: [Engine-devel] Adding atomic restore snapshot command at backend
> 
> 
> Hi Livnat,
> 
> On 05/28/2012 02:07 PM, Livnat Peer wrote:
> > On 28/05/12 12:59, Michael Pasternak wrote:
> >>
> >> Currently 'restore snapshot' done in two steps on a client side:
> >>
> >> 1. TryBackToAllSnapshotsOfVm
> >> 2. RestoreAllSnapshots
> >>
> >> This implementation creates race condition on 1 and therefore
> >> unstable & bug prone,

IIRC, there is an easy way to fix this,
by polling on engine jobs and not vdsm tasks, no?

> >> i suggested refactoring 2 to include 1 as single atomic operation
> >> at backend.
> >>
> >>
> > 
> > Hi Michael,
> > 
> > The two commands above are used for functionality that is needed
> > regardless of the functionality you are looking for.
> > We want to present the user the option to preview a snapshot
> > without
> > committing to it and without loosing the current snapshot.
> > I think we need to model the the two options above in the REST, it
> > is
> > functionality that is used in the UI.

+1 for that.

> 
> this is out of scope of this RFE.
> 
> > 
> > What you are looking for is a functionality 'restore to snapshot',
> > this
> > functionality does not exist in the engine in a single step, and I
> > think
> > that because we assumed that the user can get it in two steps It
> > wasn't
> > prioritize so far.
> 
> the cons. of this approach is an async nature of the former command.
> 
> > 
> > Implementing the missing functionality with the two functions above
> > is
> > a compromise that was done in the API.
> > 
> > To summarize I think the requirement you present is a missing
> > functionality in the engine and solving it is not about refactoring
> > the
> > two existing verbs into one.
> 
> what i meant is not deprecating/refactoring old commands, but
> introducing
> new one that reusing them and expose as single (atomic) command
> called
> 'RestoreAllSnapshots',
> 
> sorry if i wasn't clear.

another suggestion is to have a way to send to the engine list of commands that will be performed.

> 
> > 
> > Thanks, Livnat
> > 
> 
> 
> --
> 
> Michael Pasternak
> RedHat, ENG-Virtualization R&D
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