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From: Stefan Hajnoczi <stefanha@gmail.com>
Date: Thu, 16 Nov 2017 17:20:30 +0000
Message-ID: <CAJSP0QXepBMucAFNSbihp6awkurZ=47fTwgv7UHQ3MRiATGS5w@mail.gmail.com>
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Subject: Re: [Qemu-devel] Effect of qemu-img convert -m and -W options
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To: "Richard W.M. Jones" <rjones@redhat.com>
Cc: Peter Lieven <pl@kamp.de>, libguestfs@redhat.com, qemu-devel <qemu-devel@nongnu.org>

On Thu, Nov 16, 2017 at 3:12 PM, Richard W.M. Jones <rjones@redhat.com> wrote:
> On Thu, Nov 16, 2017 at 03:51:29PM +0100, Peter Lieven wrote:
>> Afaik all writes to the same QCOW2 serialize because of the s->lock that
>> is held during the write. So its not suprising that there is no benefit from
>> mutliple threads as long as reading from the RAW file involves no delay.
>> Which is likely due to readahead of the OS.
>
> I guess this would explain it, thanks.

The theory is false for preallocated qcow2 image files.  Parallel
writes do occur in block/qcow2.c:qcow2_co_pwritev() if the cluster is
already allocated:

            qemu_co_mutex_unlock(&s->lock);  <----- parallel!
            BLKDBG_EVENT(bs->file, BLKDBG_WRITE_AIO);
            trace_qcow2_writev_data(qemu_coroutine_self(),
                                    cluster_offset + offset_in_cluster);
            ret = bdrv_co_pwritev(bs->file,
                                  cluster_offset + offset_in_cluster,
                                  cur_bytes, &hd_qiov, 0);
            qemu_co_mutex_lock(&s->lock);

Stefan