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

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

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.

Den