Re: [Qemu-devel] [PATCH v2 31/32] qcow2: Allow configuring the L2 slice size

[Eric Blake <eblake@redhat.com>]

[-- Attachment #1: Type: text/plain, Size: 1903 bytes --]

On 12/15/2017 06:53 AM, Alberto Garcia wrote:
> Now that the code is ready to handle L2 slices we can finally add an
> option to allow configuring their size.
> 
> An L2 slice is the portion of an L2 table that is read by the qcow2
> cache. Until now the cache was always reading full L2 tables, and
> since the L2 table size is equal to the cluster size this was not very
> efficient with large clusters. Here's a more detailed explanation of
> why it makes sense to have smaller cache entries in order to load L2
> data:
> 
>    https://lists.gnu.org/archive/html/qemu-block/2017-09/msg00635.html
> 
> This patch introduces a new command-line option to the qcow2 driver
> named l2-cache-entry-size (cf. l2-cache-size). The cache entry size
> has the same restrictions as the cluster size: it must be a power of
> two and it has the same range of allowed values, with the additional
> requirement that it must not be larger than the cluster size.
> 
> The L2 cache entry size (L2 slice size) remains equal to the cluster
> size for now by default, so this feature must be explicitly enabled.
> Although my tests show that 4KB slices consistently improve
> performance and give the best results, let's wait and make more tests
> with different cluster sizes before deciding on an optimal default.
> 
> Signed-off-by: Alberto Garcia <berto@igalia.com>
> ---
>  block/qcow2-cache.c | 10 ++++++++--
>  block/qcow2.c       | 33 +++++++++++++++++++++++++++------
>  block/qcow2.h       |  4 +++-
>  3 files changed, 38 insertions(+), 9 deletions(-)

Is there a QMP counterpart to the command-line option?  I suspect
Kevin's work on making a QMP command for image creation will also be
impacted.

I haven't reviewed the patch closely, yet.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org


[-- Attachment #2: OpenPGP digital signature --]
[-- Type: application/pgp-signature, Size: 619 bytes --]