Re: [Qemu-devel] [PATCH] qcow2: Fix the calculation of the maximum L2 cache size

[Alberto Garcia <berto@igalia.com>]

Cc qemu-stable

This bug means that under certain conditions it's impossible to
create a cache large enough for the image, resulting in reduced I/O
performance.

On Fri, Aug 16, 2019 at 03:17:42PM +0300, 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 <berto@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);
> -- 
> 2.20.1