[libvirt] [PATCHv2 10/15] blockjob: support pivot operation on cancel

[Eric Blake]

On 04/06/2012 09:19 AM, Paolo Bonzini wrote:
> Il 06/04/2012 06:36, Eric Blake ha scritto:
>> if
>> 'block_job_cancel' were made part of 'transaction', you could
>> copy multiple disks at the same point in time without pausing
>> the domain.
> 
> This is doable.
> 
> The transactioned command would do a qemu_aio_flush() in the prepare
> phase, and a normal block_job_cancel in the commit phase.
> 
>> This also implies we may want to add a --quiesce
>> flag to the pivot operation, so that when breaking a mirror, the
>> side of the mirror that we are abandoning is at least in a stable
>> state with regards to guest I/O.
> 
> I don't think this is needed.  Either it makes a difference so it should
> be the default, or it doesn't and then it is not a useful knob.

I'm talking about the guest agent.  It may make a difference, but cannot
be the default, because you cannot trust the guest agent to be present.
 I'm thinking this will be like the --quiesce operation of
snapshot-create, as another situation where the guest agent quiesce can
usefully surround a 'transaction' command to improve the results, but
must not be default.

-- 
Eric Blake   eblake at redhat.com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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