dm snapshot: merge consecutive chunks together

s->store->type->prepare_merge returns the number of chunks that can be
copied linearly working backwards from the returned chunk number.

For example, if it returns 3 chunks with old_chunk == 10 and new_chunk
== 20, then chunk 20 can be copied to 10, chunk 19 to 9 and 18 to 8.

Until now kcopyd only copied one chunk at a time.  This patch now copies
the full set at once.

Consequently, snapshot_merge_process() needs to delay the merging of all
chunks if any have writes in progress, not just the first chunk in the
region that is to be merged.

snapshot-merge's performance is now comparable to the original
snapshot-origin target.

Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>

diff --git a/drivers/md/dm-snap.c b/drivers/md/dm-snap.c
index 1498704..bb4b733 100644 (file)
--- a/drivers/md/dm-snap.c
+++ b/drivers/md/dm-snap.c
@@ -879,9 +879,10 @@ static void increment_pending_exceptions_done_count(void)
 
 static void snapshot_merge_next_chunks(struct dm_snapshot *s)
 {
-       int r;
+       int i, linear_chunks;
        chunk_t old_chunk, new_chunk;
        struct dm_io_region src, dest;
+       sector_t io_size;
        uint64_t previous_count;
 
        BUG_ON(!test_bit(RUNNING_MERGE, &s->state_bits));
@@ -896,20 +897,28 @@ static void snapshot_merge_next_chunks(struct dm_snapshot *s)
                goto shut;
        }
 
-       r = s->store->type->prepare_merge(s->store, &old_chunk, &new_chunk);
-       if (r <= 0) {
-               if (r < 0)
+       linear_chunks = s->store->type->prepare_merge(s->store, &old_chunk,
+                                                     &new_chunk);
+       if (linear_chunks <= 0) {
+               if (linear_chunks < 0)
                        DMERR("Read error in exception store: "
                              "shutting down merge");
                goto shut;
        }
 
-       /* TODO: use larger I/O size once we verify that kcopyd handles it */
+       /* Adjust old_chunk and new_chunk to reflect start of linear region */
+       old_chunk = old_chunk + 1 - linear_chunks;
+       new_chunk = new_chunk + 1 - linear_chunks;
+
+       /*
+        * Use one (potentially large) I/O to copy all 'linear_chunks'
+        * from the exception store to the origin
+        */
+       io_size = linear_chunks * s->store->chunk_size;
 
        dest.bdev = s->origin->bdev;
        dest.sector = chunk_to_sector(s->store, old_chunk);
-       dest.count = min((sector_t)s->store->chunk_size,
-                        get_dev_size(dest.bdev) - dest.sector);
+       dest.count = min(io_size, get_dev_size(dest.bdev) - dest.sector);
 
        src.bdev = s->cow->bdev;
        src.sector = chunk_to_sector(s->store, new_chunk);
@@ -925,7 +934,7 @@ static void snapshot_merge_next_chunks(struct dm_snapshot *s)
         * significant impact on performance.
         */
        previous_count = read_pending_exceptions_done_count();
-       while (origin_write_extent(s, dest.sector, s->store->chunk_size)) {
+       while (origin_write_extent(s, dest.sector, io_size)) {
                wait_event(_pending_exceptions_done,
                           (read_pending_exceptions_done_count() !=
                            previous_count));
@@ -935,10 +944,12 @@ static void snapshot_merge_next_chunks(struct dm_snapshot *s)
 
        down_write(&s->lock);
        s->first_merging_chunk = old_chunk;
-       s->num_merging_chunks = 1;
+       s->num_merging_chunks = linear_chunks;
        up_write(&s->lock);
 
-       __check_for_conflicting_io(s, old_chunk);
+       /* Wait until writes to all 'linear_chunks' drain */
+       for (i = 0; i < linear_chunks; i++)
+               __check_for_conflicting_io(s, old_chunk + i);
 
        dm_kcopyd_copy(s->kcopyd_client, &src, 1, &dest, 0, merge_callback, s);
        return;