Re: [Qemu-devel] [RFC PATCH 0/3] Point-in-time snapshot exporting with drive-backup

[Fam Zheng]

On Wed, 06/26 09:23, Paolo Bonzini wrote:
> Il 26/06/2013 05:59, Fam Zheng ha scritto:
> > 
> > The block job originally had total control over target bs: when the job
> > completes, the target is deleted. Now it's shared with NBD, so we should 
> > make
> > sure the job completion or canceling wouldn't crash NBD server. This patch
> > doesn't handle this case: if the block job is ended before NBD exporting,
> > there'll be problem.
> 
> This should not be a problem.  When the job completes or cancels it will
> close s->target.  The NBD server has installed a close notifier for
> s->target, and will close connections with the clients.
> 
> Similarly, if the source disk is hot-unplugged, the bdrv_close will
> cancel the job, which in turn closes connections with the clients.
> 
> So this just works as long as no hot-unplug happens and as long as the
> job doesn't complete: the client can use the point-in-time snapshot via
> NBD as long as it cares to.

OK, thanks for pointing this out.

> 
> This leads to another observation: a sync:'none' block-backup job
> probably should never complete, and should instead go on until explicit
> cancellation.  This is because the job does no background writes, and
> thus completion would only happen after the guest has written the whole
> disk.  Writing the whole disk is rare enough that it will likely cause
> bugs in the clients.  It is easier just to never complete the job.
> 

Yes, the sync mode none will simply run forever until cancelled.

-- 
Fam