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From: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-ID: <57EE9CA4.6010801@virtuozzo.com>
Date: Fri, 30 Sep 2016 20:11:00 +0300
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Subject: [Qemu-devel] backup notifier fail policy
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To: qemu-devel <qemu-devel@nongnu.org>, qemu block <qemu-block@nongnu.org>
Cc: Jeff Cody <jcody@redhat.com>, John Snow <jsnow@redhat.com>, Fam Zheng <famz@redhat.com>, Stefan Hajnoczi <stefanha@redhat.com>, "Denis V. Lunev" <den@openvz.org>

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?

-- 
Best regards,
Vladimir