On 08.05.2017 16:15, Stefan Hajnoczi wrote:
> The refcount metadata size calculation is inaccurate and can produce
> numbers that are too small. This is bad because we should calculate a
> conservative number - one that is guaranteed to be large enough.
>
> This patch switches the approach to a fixed point calculation because
> the existing equation is hard to solve when inaccuracies are taken care
> of.
>
> Signed-off-by: Stefan Hajnoczi <stefa...@redhat.com>
> Reviewed-by: Alberto Garcia <be...@igalia.com>
> ---
> block/qcow2.c | 82
> ++++++++++++++++++++++++++++++-----------------------------
> 1 file changed, 42 insertions(+), 40 deletions(-)
>
> diff --git a/block/qcow2.c b/block/qcow2.c
> index 5569b63..ff0d825 100644
> --- a/block/qcow2.c
> +++ b/block/qcow2.c
> @@ -2095,6 +2095,43 @@ static int preallocate(BlockDriverState *bs)
> return 0;
> }
>
> +/* qcow2_refcount_metadata_size:
> + * @clusters: number of clusters to refcount (including data and L1/L2
> tables)
> + * @cluster_size: size of a cluster, in bytes
> + * @refcount_order: refcount bits power-of-2 exponent
> + *
> + * Returns: Number of bytes required for refcount blocks and table metadata.
> + */
> +static int64_t qcow2_refcount_metadata_size(int64_t clusters,
> + size_t cluster_size,
> + int refcount_order)
> +{
> + /*
> + * Every host cluster is reference-counted, including metadata (even
> + * refcount metadata is recursively included).
> + *
> + * An accurate formula for the size of refcount metadata size is
> difficult
> + * to derive.Oh, by the way: https://lists.nongnu.org/archive/html/qemu-devel/2014-04/msg04820.html *cough* *cough* (No, this is not the formula that was used for this preallocation. Otherwise, it would have been correct. O:-)) Max > An easier method of calculation is finding the fixed point > + * where no further refcount blocks or table clusters are required to > + * reference count every cluster. > + */ > + int64_t blocks_per_table_cluster = cluster_size / sizeof(uint64_t); > + int64_t refcounts_per_block = cluster_size * 8 / (1 << refcount_order); > + int64_t table = 0; /* number of refcount table clusters */ > + int64_t blocks = 0; /* number of refcount block clusters */ > + int64_t last; > + int64_t n = 0; > + > + do { > + last = n; > + blocks = DIV_ROUND_UP(clusters + table + blocks, > refcounts_per_block); > + table = DIV_ROUND_UP(blocks, blocks_per_table_cluster); > + n = clusters + blocks + table; > + } while (n != last); > + > + return (blocks + table) * cluster_size; > +} > + > /** > * qcow2_calc_prealloc_size: > * @total_size: virtual disk size in bytes
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