From: Alberto Garcia <berto@igalia.com>
To: qemu-devel@nongnu.org
Cc: Kevin Wolf <kwolf@redhat.com>,
Leonid Bloch <lbloch@janustech.com>,
qemu-stable@nongnu.org, qemu-block@nongnu.org,
Max Reitz <mreitz@redhat.com>
Subject: Re: [Qemu-devel] [PATCH] qcow2: Fix the calculation of the maximum L2 cache size
Date: Fri, 16 Aug 2019 14:41:27 +0200 [thread overview]
Message-ID: <20190816124127.GA20917@igalia.com> (raw)
In-Reply-To: <20190816121742.29607-1-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
next prev parent reply other threads:[~2019-08-16 12:42 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-08-16 12:17 [Qemu-devel] [PATCH] qcow2: Fix the calculation of the maximum L2 cache size Alberto Garcia
2019-08-16 12:41 ` Alberto Garcia [this message]
2019-08-16 12:59 ` Kevin Wolf
2019-08-16 13:30 ` Alberto Garcia
2019-08-16 14:08 ` Kevin Wolf
2019-08-16 14:25 ` Alberto Garcia
2019-08-18 10:17 ` Leonid Bloch
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