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

["Denis V. Lunev" <den@openvz.org>]

On 04/27/2016 10:59 AM, Fam Zheng wrote:
> 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
>
ya, you are correct here.

This is definitely missed.

Thank you.

Den