Commit 60e356f3 authored Sep 18, 2013 by Mikulas Patocka Committed by Mike Snitzer Sep 20, 2013

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

parent 5ea330a7

drivers/md/dm-snap.c

+2 −3

Original line number	Diff line number	Diff line
		@@ -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)

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