Re: [Qemu-block] backup notifier fail policy

[Stefan Hajnoczi]

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.

Stefan

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