Re: [PATCH 0/1] qcow2: Skip copy-on-write when allocating a zero cluster

[Vladimir Sementsov-Ogievskiy]

Hi!

14.08.2020 17:57, Alberto Garcia wrote:
> Hi,
>
> the patch is self-explanatory, but I'm using the cover letter to raise
> a couple of related questions.
>
> Since commit c8bb23cbdbe / QEMU 4.1.0 (and if the storage backend
> allows it) writing to an image created with preallocation=metadata can
> be slower (20% in my tests) than writing to an image with no
> preallocation at all.
>
> So:
>
> a) shall we include a warning in the documentation ("note that this
>     preallocation mode can result in worse performance")?

I think, the best thing to do is to make it work fast in all cases if possible 
(I assume, that would be, with your patch + positive answer to [b]? Or not?) :)

Andrey recently added a benchmark, with some cases, where c8bb23cbdbe bring 
benefits:
[PATCH v6] scripts/simplebench: compare write request performance
<1594741846-475697-1-git-send-email-andrey.shinkevich@virtuozzo.com>
queued in Eduardo's python-next: 
https://github.com/ehabkost/qemu/commit/9519f87d900b0ef30075c749fa097bd93471553f

So, as a first step, could you post your tests, so we can add it into this 
benchmark? Or post a patch to simplebench on top of Eduardo's python-next.

> b) why don't we also initialize preallocated clusters with
>     QCOW_OFLAG_ZERO? (at least when there's no subclusters involved,
>     i.e. no backing file). This would make reading from them (and
>     writing to them, after this patch) faster.

Probably, they are not guaranteed to be zero on all filesystems? But I think at 
least in some cases (99% :) we can mark them as ZERO.. Honestly, I may be not 
aware of actual reasons.

> Berto
>
> Alberto Garcia (1):
>    qcow2: Skip copy-on-write when allocating a zero cluster
>
>   include/block/block.h |  2 +-
>   block/commit.c        |  2 +-
>   block/io.c            | 20 +++++++++++++++++---
>   block/mirror.c        |  3 ++-
>   block/qcow2.c         | 26 ++++++++++++++++----------
>   block/replication.c   |  2 +-
>   block/stream.c        |  2 +-
>   qemu-img.c            |  2 +-
>   8 files changed, 40 insertions(+), 19 deletions(-)


--
Best regards,
Vladimir