[ 3.8.y.z extended stable ] Patch "dm-snapshot: fix performance degradation due to small hash size" has been added to staging queue

[Kamal Mostafa]

This is a note to let you know that I have just added a patch titled

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

to the linux-3.8.y-queue branch of the 3.8.y.z extended stable tree 
which can be found at:

 http://kernel.ubuntu.com/git?p=ubuntu/linux.git;a=shortlog;h=refs/heads/linux-3.8.y-queue

This patch is scheduled to be released in version 3.8.13.11.

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.8.y.z tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Kamal

------

>From b5b564c1a1e7fcd9cbc50bf44a8dc74d41019355 Mon Sep 17 00:00:00 2001
From: Mikulas Patocka <mpatocka at redhat.com>
Date: Wed, 18 Sep 2013 19:40:42 -0400
Subject: dm-snapshot: fix performance degradation due to small hash size

commit 60e356f381954d79088d0455e357db48cfdd6857 upstream.

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 at redhat.com>
Signed-off-by: Mike Snitzer <snitzer at redhat.com>
Signed-off-by: Kamal Mostafa <kamal at canonical.com>
---
 drivers/md/dm-snap.c | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/drivers/md/dm-snap.c b/drivers/md/dm-snap.c
index 900eb23..8b36c52 100644
--- a/drivers/md/dm-snap.c
+++ b/drivers/md/dm-snap.c
@@ -723,17 +723,16 @@ 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)
--
1.8.1.2