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Re: [Qemu-block] [Qemu-devel] [PATCH v2 31/32] qcow2: Allow configuring
Re: [Qemu-block] [Qemu-devel] [PATCH v2 31/32] qcow2: Allow configuring the L2 slice size
Tue, 16 Jan 2018 17:18:25 -0600
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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
> 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 <address@hidden>
> 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
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
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