dm-snapshot: fix performance degradation due to small hash size

LVM2, since version 2.02.96, creates origin with zero size, then loads
the snapshot driver and then loads the origin.  Consequently, the
snapshot driver sees the origin size zero and sets the hash size to the
lower bound 64.  Such small hash table causes performance degradation.

This patch changes it so that the hash size is determined by the size of
snapshot volume, not minimum of origin and snapshot size.  It doesn't
make sense to set the snapshot size significantly larger than the origin
size, so we do not need to take origin size into account when
calculating the hash size.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Cc: stable@vger.kernel.org

diff --git a/drivers/md/dm-snap.c b/drivers/md/dm-snap.c
index c434e5aab2dfc9e6a63ca7700e5ac1c1deacd025..aec57d76db5d616c8e692fa95cee58a8f62a0573 100644 (file)
--- a/drivers/md/dm-snap.c
+++ b/drivers/md/dm-snap.c
@@ -725,17 +725,16 @@ static int calc_max_buckets(void)
  */
 static int init_hash_tables(struct dm_snapshot *s)
 {
-       sector_t hash_size, cow_dev_size, origin_dev_size, max_buckets;
+       sector_t hash_size, cow_dev_size, max_buckets;
 
        /*
         * Calculate based on the size of the original volume or
         * the COW volume...
         */
        cow_dev_size = get_dev_size(s->cow->bdev);
-       origin_dev_size = get_dev_size(s->origin->bdev);
        max_buckets = calc_max_buckets();
 
-       hash_size = min(origin_dev_size, cow_dev_size) >> s->store->chunk_shift;
+       hash_size = cow_dev_size >> s->store->chunk_shift;
        hash_size = min(hash_size, max_buckets);
 
        if (hash_size < 64)