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

[John Snow]

On 10/04/2016 12:02 PM, Stefan Hajnoczi wrote:
> On Tue, Oct 04, 2016 at 01:55:30PM +0200, Kevin Wolf wrote:
> > Am 04.10.2016 um 12:41 hat Denis V. Lunev geschrieben:
> > > On 10/04/2016 12:34 PM, Kevin Wolf wrote:
> > > > 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
> > > In this case policy for guest error and policy for backup
> > > error should be different policies or I have missed something.
> >
> > I guess so.
>
> There are separate error policies for -device and the blockjob.  Perhaps
> the blockjob error policy can be used in the write notifier code path if
> the failure occurs while writing to the backup target.
>
> Stefan

Sounds good to me.