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

[Alberto Garcia <berto@igalia.com>]

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")?

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.

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(-)

-- 
2.20.1