Re: [Qemu-block] [PATCH v3 36/39] qcow2: Allow configuring the L2 slice size

[Max Reitz]

On 2018-01-26 16:00, 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.
> 
> Now that the cache entry size is not necessarily equal to the cluster
> size we need to reflect that in the MIN_L2_CACHE_SIZE documentation.
> That minimum value is a requirement of the COW algorithm: we need to
> read two L2 slices (and not two L2 tables) in order to do COW, see
> l2_allocate() for the actual code.
> 
> Signed-off-by: Alberto Garcia <address@hidden>
> ---
>  block/qcow2-cache.c  | 10 ++++++++--
>  block/qcow2.c        | 34 +++++++++++++++++++++++++++-------
>  block/qcow2.h        |  6 ++++--
>  qapi/block-core.json |  6 ++++++
>  4 files changed, 45 insertions(+), 11 deletions(-)

Reviewed-by: Max Reitz <address@hidden>

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