From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:48862) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1brOJy-0002Oi-F4 for qemu-devel@nongnu.org; Tue, 04 Oct 2016 07:55:43 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1brOJw-0006kn-6L for qemu-devel@nongnu.org; Tue, 04 Oct 2016 07:55:41 -0400 Date: Tue, 4 Oct 2016 13:55:30 +0200 From: Kevin Wolf Message-ID: <20161004115530.GE5316@noname.str.redhat.com> References: <57EE9CA4.6010801@virtuozzo.com> <57EEB604.1090908@virtuozzo.com> <20161003131151.GB31993@stefanha-x1.localdomain> <20161004092349.GD4587@stefanha-x1.localdomain> <20161004093418.GC5316@noname.str.redhat.com> <790008a1-96a4-b68c-2463-fa817dcdb607@openvz.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <790008a1-96a4-b68c-2463-fa817dcdb607@openvz.org> Subject: Re: [Qemu-devel] backup notifier fail policy List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: "Denis V. Lunev" Cc: Stefan Hajnoczi , John Snow , Vladimir Sementsov-Ogievskiy , Fam Zheng , qemu block , Jeff Cody , qemu-devel 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