Am 16.08.2019 um 14:17 hat Alberto Garcia geschrieben: > The size of the qcow2 L2 cache defaults to 32 MB, which can be easily > larger than the maximum amount of L2 metadata that the image can have. > For example: with 64 KB clusters the user would need a qcow2 image > with a virtual size of 256 GB in order to have 32 MB of L2 metadata. > > Because of that, since commit b749562d9822d14ef69c9eaa5f85903010b86c30 > we forbid the L2 cache to become larger than the maximum amount of L2 > metadata for the image, calculated using this formula: > > uint64_t max_l2_cache = virtual_disk_size / (s->cluster_size / 8); > > The problem with this formula is that the result should be rounded up > to the cluster size because an L2 table on disk always takes one full > cluster. > > For example, a 1280 MB qcow2 image with 64 KB clusters needs exactly > 160 KB of L2 metadata, but we need 192 KB on disk (3 clusters) even if > the last 32 KB of those are not going to be used. > > However QEMU rounds the numbers down and only creates 2 cache tables > (128 KB), which is not enough for the image. > > A quick test doing 4KB random writes on a 1280 MB image gives me > around 500 IOPS, while with the correct cache size I get 16K IOPS. > > Signed-off-by: Alberto Garcia <be...@igalia.com>
Hm, this is bad. :-( The requirement so that this bug doesn't affect the user seems to be that the image size is a multiple of 64k * 8k = 512 MB. Which means that users are probably often lucky enough in practice. I'll Cc: qemu-stable anyway. Thanks, applied to the block branch. Kevin
