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    Subject[PATCH 058/104] dm-snapshot: fix performance degradation due to small hash size
    Date
    3.8.13.11 -stable review patch.  If anyone has any objections, please let me know.

    ------------------

    From: Mikulas Patocka <mpatocka@redhat.com>

    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@redhat.com>
    Signed-off-by: Mike Snitzer <snitzer@redhat.com>
    Signed-off-by: Kamal Mostafa <kamal@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


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