Re: [Qemu-devel] [PATCH for 2.7 0/2] backup compression

[Fam Zheng <famz@redhat.com>]

On Wed, 04/27 09:32, Denis V. Lunev wrote:
> On 04/26/2016 12:46 PM, Stefan Hajnoczi wrote:
> > On Sat, Apr 23, 2016 at 11:41:51AM +0300, Denis V. Lunev wrote:
> > > The idea is simple - backup is "written-once" data. It is written block
> > > by block and it is large enough. It would be nice to save storage
> > > space and compress it.
> > > 
> > > These patches add the ability to compress data during backup. This
> > > functionality is implemented by means of adding options to the qmp/hmp
> > > commands(drive-backup, blockdev-backup). The implementation is quite
> > > simple, because the responsibility for data compression imposed on the
> > > format driver.
> > > 
> > > Signed-off-by: Pavel Butsykin <pbutsykin@virtuozzo.com>
> > > Signed-off-by: Denis V. Lunev <den@openvz.org>
> > > CC: Jeff Cody <jcody@redhat.com>
> > > CC: Markus Armbruster <armbru@redhat.com>
> > > CC: Eric Blake <eblake@redhat.com>
> > > CC: John Snow <jsnow@redhat.com>
> > > 
> > > Pavel Butsykin (2):
> > >    drive-backup: added support for data compression
> > >    blockdev-backup: added support for data compression
> > > 
> > >   block/backup.c            | 13 +++++++++++++
> > >   blockdev.c                | 20 ++++++++++++++++++--
> > >   hmp-commands.hx           |  8 +++++---
> > >   hmp.c                     |  3 ++-
> > >   include/block/block_int.h |  1 +
> > >   qapi/block-core.json      |  3 ++-
> > >   qmp-commands.hx           |  7 +++++--
> > >   7 files changed, 46 insertions(+), 9 deletions(-)
> > bdrv_write_compressed() hasn't been called from a running VM before, it
> > was purely a synchronous qemu-img operation.  The
> > qcow2.c:qcow2_write_compressed() code doesn't seem to be written with
> > coroutine context in mind but I think the lock in
> > block/backup.c:backup_do_cow() will protect against race conditions.
> > 
> > Please include a test case (using qcow2?).
> I have looked into the code. The only problematic thing there
> is that the compression is performed directly in the write,
> which could take a bit of time.
> 
> Thus responsiveness of the guest system could be reduced.
> Though I do not expect here troubles. I think that it is worth
> to give this tech a chance. Compression could be moved to
> the outer thread in the next step if required.

As far as I can tell, in order to be used in a coroutine, we need a
"qemu_co_mutex_lock(&s->lock);" pair in vmdk_write_compressed. Something
similar probably apply to qcow2 as well.

Maybe what we should do is adding a bdrv_co_write_compressed().

Fam