Re: [Qemu-block] [Qemu-devel] backup notifier fail policy

[Kevin Wolf]

Am 04.10.2016 um 11:23 hat Stefan Hajnoczi geschrieben:
> On Mon, Oct 03, 2016 at 02:07:34PM -0400, John Snow wrote:
> > 
> > 
> > On 10/03/2016 09:11 AM, Stefan Hajnoczi wrote:
> > > On Fri, Sep 30, 2016 at 09:59:16PM +0300, Vladimir Sementsov-Ogievskiy 
> > > wrote:
> > > > On 30.09.2016 20:11, Vladimir Sementsov-Ogievskiy wrote:
> > > > > Hi all!
> > > > > 
> > > > > Please, can somebody explain me, why we fail guest request in case of 
> > > > > io
> > > > > error in write notifier? I think guest consistency is more important
> > > > > than success of unfinished backup. Or, what am I missing?
> > > > > 
> > > > > I'm saying about this code:
> > > > > 
> > > > > static int coroutine_fn backup_before_write_notify(
> > > > >         NotifierWithReturn *notifier,
> > > > >         void *opaque)
> > > > > {
> > > > >     BackupBlockJob *job = container_of(notifier, BackupBlockJob,
> > > > > before_write);
> > > > >     BdrvTrackedRequest *req = opaque;
> > > > >     int64_t sector_num = req->offset >> BDRV_SECTOR_BITS;
> > > > >     int nb_sectors = req->bytes >> BDRV_SECTOR_BITS;
> > > > > 
> > > > >     assert(req->bs == blk_bs(job->common.blk));
> > > > >     assert((req->offset & (BDRV_SECTOR_SIZE - 1)) == 0);
> > > > >     assert((req->bytes & (BDRV_SECTOR_SIZE - 1)) == 0);
> > > > > 
> > > > >     return backup_do_cow(job, sector_num, nb_sectors, NULL, true);
> > > > > }
> > > > > 
> > > > > So, what about something like
> > > > > 
> > > > > ret = backup_do_cow(job, ...
> > > > > if (ret < 0 && job->notif_ret == 0) {
> > > > >    job->notif_ret = ret;
> > > > > }
> > > > > 
> > > > > return 0;
> > > > > 
> > > > > and fail block job if notif_ret < 0 in other places of backup code?
> > > > > 
> > > > 
> > > > And second question about notifiers in backup block job. If block job is
> > > > paused, notifiers still works and can copy data. Is it ok? So, user 
> > > > thinks
> > > > that job is paused, so he can do something with target disk.. But 
> > > > really,
> > > > this 'something' will race with write-notifiers. So, what assumptions 
> > > > may
> > > > user actually have about paused backup job? Is there any agreements? 
> > > > Also,
> > > > on query-block-jobs we will see job.busy = false, when actually
> > > > copy-on-write may be in flight..
> > > 
> > > I agree that the job should fail and the guest continues running.
> > > 
> > > The backup job cannot do the usual ENOSPC stop/resume error handling
> > > since we lose snapshot consistency once guest writes are allowed to
> > > proceed.  Backup errors need to be fatal, resuming is usually not
> > > possible.  The user will have to retry the backup operation.
> > > 
> > > Stefan
> > > 
> > 
> > If we fail and intercept the error for the backup write and HALT at that
> > point, why would we lose consistency? If the backup write failed before we
> > allowed the guest write to proceed, that data should still be there on disk,
> > no?
> 
> I missed that there are two separate error handling approaches used in
> block/backup.c:
> 
> 1. In the write notifier I/O errors are treated as if the guest write
> failed.
> 
> 2. In the backup_run() loop I/O errors affect the block job's error
> status.
> 
> I was thinking of case #2 instead of case #1.
> 
> > Eh, regardless: If we're not using a STOP policy, it seems like the right
> > thing to do is definitely to just fail the backup instead of failing the
> > write.
> 
> Even with a -drive werror=stop policy the user probably doesn't want
> guest downtime if writing to the backup target fails.

That's a policy decision that ultimately only the user can make. For one
user, it might be preferable to cancel the backup and keep the VM
running, but for another user it may be more important to keep a
consistent snapshot of the point in time when the backup job was started
than keeping the VM running.

Kevin

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