Re: [Qemu-devel] Re: Strategic decision: COW format

[Gleb Natapov]

On Wed, Feb 23, 2011 at 09:52:02AM -0600, Anthony Liguori wrote:
> On 02/23/2011 09:37 AM, Avi Kivity wrote:
> >On 02/23/2011 05:31 PM, Anthony Liguori wrote:
> >>>
> >>>>what about snapshots?  Are we okay having a feature in a
> >>>>prominent format that isn't going to meet user's
> >>>>expectations?
> >>>>
> >>>>Is there any hope that an image with 1000, 1000, or 10000
> >>>>snapshots is going to have even reasonable performance in
> >>>>qcow2?
> >>>>
> >>>
> >>>Are thousands of snapshots for a single image a reasonable
> >>>user expectation?  What's the use case?
> >>
> >>
> >>Checkpointing.  It was the original use-case that led to qcow2
> >>being invented.
> >
> >I still don't see.  What would you do with thousands of checkpoints?
> 
> Er, hit send to quickly.
> 
> HPC is a big space where checkpointing is actually useful.  An HPC
> workload may take weeks to run to completion.  If something fails
> during the run, it's a huge waste of time.  However, if you do
> regularl checkpointing, a failure may only lose a few minutes of
> work instead of the entire weeks worth of work.
> 
HPC workload mostly run on cluster nowadays. Getting consistent
distributed snapshot without messages in flight is not as simple as
snapshotting bunch of VMs at random time. Anyway in HPC scenario you
need only one (last) snapshot, not thousands of them.

--
                        Gleb.