Thanks, Berto!
On 8/16/19 3:17 PM, Alberto Garcia wrote: > 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> > --- > block/qcow2.c | 6 +++++- > 1 file changed, 5 insertions(+), 1 deletion(-) > > diff --git a/block/qcow2.c b/block/qcow2.c > index 039bdc2f7e..865839682c 100644 > --- a/block/qcow2.c > +++ b/block/qcow2.c > @@ -826,7 +826,11 @@ static void read_cache_sizes(BlockDriverState *bs, > QemuOpts *opts, > bool l2_cache_entry_size_set; > int min_refcount_cache = MIN_REFCOUNT_CACHE_SIZE * s->cluster_size; > uint64_t virtual_disk_size = bs->total_sectors * BDRV_SECTOR_SIZE; > - uint64_t max_l2_cache = virtual_disk_size / (s->cluster_size / 8); > + uint64_t max_l2_entries = DIV_ROUND_UP(virtual_disk_size, > s->cluster_size); > + /* An L2 table is always one cluster in size so the max cache size > + * should be a multiple of the cluster size. */ > + uint64_t max_l2_cache = ROUND_UP(max_l2_entries * sizeof(uint64_t), > + s->cluster_size); > > combined_cache_size_set = qemu_opt_get(opts, QCOW2_OPT_CACHE_SIZE); > l2_cache_size_set = qemu_opt_get(opts, QCOW2_OPT_L2_CACHE_SIZE); >
