Re: [Libguestfs] [Qemu-devel] Effect of qemu-img convert -m and -W options
On Thu, Nov 16, 2017 at 3:12 PM, Richard W.M. Jones <rjones(a)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