Am 13.02.2019 um 17:48 hat Alberto Garcia geschrieben: > QEMU 2.12 (commit 1221fe6f636754ab5f2c1c87caa77633e9123622) introduced > a new setting called l2-cache-entry-size that allows making entries on > the qcow2 L2 cache smaller than the cluster size. > > I have been performing several tests with different cluster and entry > sizes and all of them show that reducing the entry size (aka L2 slice) > consistently improves I/O performance, notably during random I/O (all > tests done with sequential I/O show similar results). This is to be > expected because loading and evicting an L2 slice is more expensive > the larger the slice is. > > Here are some numbers on fully populated 40GB qcow2 images. The > rightmost column represents the maximum L2 cache size in both cases. > > Cluster size = 64 KB > |-------------+--------------+--------------+--------------| > | | 1MB L2 cache | 3MB L2 cache | 5MB L2 cache | > |-------------+--------------+--------------+--------------| > | 4KB slices | 6545 IOPS | 12045 IOPS | 55680 IOPS | > | 16KB slices | 5177 IOPS | 9798 IOPS | 56278 IOPS | > | 64KB slices | 2718 IOPS | 5326 IOPS | 57355 IOPS | > |-------------+--------------+--------------+--------------| > > Cluster size = 256 KB > |--------------+----------------+--------------+-----------------| > | | 512KB L2 cache | 1MB L2 cache | 1280KB L2 cache | > |--------------+----------------+--------------+-----------------| > | 4KB slices | 8539 IOPS | 21071 IOPS | 55417 IOPS | > | 64KB slices | 3598 IOPS | 9772 IOPS | 57687 IOPS | > | 256KB slices | 1415 IOPS | 4120 IOPS | 58001 IOPS | > |--------------+----------------+--------------+-----------------| > > As can be seen in the numbers, the only exception to the rule is when > the cache is large enough to hold all L2 tables. This is also to be > expected because in this case no cache entry is ever evicted so > reducing its size doesn't bring any benefit. > > This patch sets the default L2 cache entry size to 4KB except when the > cache is large enough for the whole disk. > > Signed-off-by: Alberto Garcia <be...@igalia.com>
Thanks, applied to the block branch. Kevin
