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Re: [Qemu-devel] [PATCH] [RFC] savevm only saves disk state


From: edison
Subject: Re: [Qemu-devel] [PATCH] [RFC] savevm only saves disk state
Date: Wed, 15 Sep 2010 11:24:39 -0700

On Thu, Sep 9, 2010 at 7:08 PM, Anthony Liguori <address@hidden> wrote:
> On 09/09/2010 08:43 PM, address@hidden wrote:
>>
>> From: edison<address@hidden>
>>
>> Add a new option when "savevm": savevm -n snapshotName, which only takes
>> snapshot on disk, but doesn't save vm state(memory,cpu,devices...).
>> Saving vm state on QCOW2 disk will take a long time, per my test, it will
>> take 1~2 minutes to "savevm" on VM with 1G memory. Even worse, the VM is
>> wholely stopped at that time, makes "savevm" not that useful.
>> All we know the side effect of it:) but does it make sense to give user
>> the choice?
>>
>
> I think it would be better to explore ways to make savevm live.  A round
> about option would be to combine a disk-only snapshot with a live migration
> to disk and somehow allow qcow2 to refer to an external memory snapshot.
>

My point is we still need an interface to take online snapshot for
disk-only(flush pending I/O, take QCOW2 snapshot, no savevm, no live
migration), which is the fastest , lowest down time and easy to
manage.
Look like VMware supports such kind of operation: take a snapshot
without saving 
memory(http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=1015180).

Of cause, it's better to cooperate with pv driver, or tools inside
guest, which can hold I/O, flush disk cache, etc. If without such
co-operation, the bare disk snapshot is just like saving the disk
state when loosing power, not that bad, right?

> A better alternative would be a live snapshot within qcow2.  I think Kevin
> has some good ideas about how to do this with qcow2 today but provided we
> had a nice interface to do this, the changes to the live migration code
> should be fairly straight forward.

Hi Kevin, any idea about it? Live snapshot is very useful. Do you
already have ideas/plans about it? I want to take a look at it, make
it live!


>
> Regards,
>
> Anthony Liguori
>
>



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