Am 19.02.2018 um 15:54 hat Alberto Garcia geschrieben: > This patch updates docs/qcow2-cache.txt explaining how to use the new > l2-cache-entry-size parameter. > > Here's a more detailed technical description of this feature: > > https://lists.gnu.org/archive/html/qemu-block/2017-09/msg00635.html > > And here are some performance numbers: > > https://lists.gnu.org/archive/html/qemu-block/2017-12/msg00507.html > > Signed-off-by: Alberto Garcia <be...@igalia.com>
Thanks, applied to the block branch. While reviewing this, I read the whole document and stumbled across these paragraphs: > The reason for this 1/4 ratio is to ensure that both caches cover the > same amount of disk space. Note however that this is only valid with > the default value of refcount_bits (16). If you are using a different > value you might want to calculate both cache sizes yourself since QEMU > will always use the same 1/4 ratio. Sounds like we should fix our defaults? While we're at it, would l2-cache-entry-size = MIN(cluster_size, 64k) make sense as a default? > It's also worth mentioning that there's no strict need for both caches > to cover the same amount of disk space. The refcount cache is used > much less often than the L2 cache, so it's perfectly reasonable to > keep it small. More precisely, it is only used for cluster allocation, not for read or for rewrites. Usually this means that it's indeed accessed a lot less, though especially in benchmarks, this isn't necessarily less often. However, the more important part is that even for allocating writes with random I/O, the refcount cache is still accessed sequentially and we don't really take advantage of having more than a single refcount block in memory. This only stops being true as soon as you add something that can free clusters (discards, overwriting compressed cluster, deleting internal snapshots). We have a minimum refcount block cache size of 4 clusters because of the possible recursion during refcount table growth, which leaves some room to hold the refcount block for an occasional discard (and subsequent reallocation). So should we default to this minimum on the grounds that for most people, refcounts blocks are probably only accessed sequentially in practice? The remaining memory of the total cache size seems to help the average case more if it's added to the L2 cache instead. Kevin
