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

[Kevin Wolf]

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.

Kevin