Re: [Qemu-devel] drive transactions (was Re: [PATCH 2/2 v2] Add the blockdev-reopen and blockdev-migrate commands)

[Anthony Liguori <anthony@codemonkey.ws>]

On 02/27/2012 10:51 AM, Paolo Bonzini wrote:
> On 02/27/2012 04:24 PM, Anthony Liguori wrote:
>>
>> Then you get an error with the block devices still frozen.  You can
>> execute another command to reopen back to the old image to roll back the
>> transaction.
>>
>> Pushing the rollback logic to the client does make the client interface
>> a bit more complicated and adds latency to the error path but it's much
>> easier than building a complex transaction infrastructure.
>>
>> And there are examples of this in the wild too.  LDAP uses a similar
>> mechanism.
>
> Actually, have you seen Jeff's atomic snapshot patch?  It really
> implements all that is needed to do transactions, and gets 100% ok
> error-recovery unlike the existing blockdev_snapshot_sync.  It really
> looks like we can do better than client-side error management, and there
> is not that much complexity at all.
>
> Jeff could rework his patches to work with transaction begin/commit
> commands, and Federico can then add drive-reopen and drive-migrate on top.

Yes, maybe I lack imagination but I fail to see how it generalizes easily/nicely.

 From what I can tell, all of the rollback logic is very specific to the 
commands being used, right?

Regards,

Anthony Liguori

>
> Paolo