dm exception store: add merge specific methods

Add functions that decide how many consecutive chunks of snapshot to
merge back into the origin next and to update the metadata afterwards.

prepare_merge provides a pointer to the most recent still-to-be-merged
chunk and returns how many previous ones are consecutive and can be
processed together.

commit_merge removes the nr_merged most-recent chunks permanently from
the exception store.  The number must not exceed that returned by
prepare_merge.

Introduce NUM_SNAPSHOT_HDR_CHUNKS to show where the snapshot header
chunk is accounted for.

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

1 files changed, 17 insertions, 0 deletions

diff --git a/drivers/md/dm-exception-store.h b/drivers/md/dm-exception-store.h
index bb8874653de1..c53e08935b42 100644
--- a/drivers/md/dm-exception-store.h
+++ b/drivers/md/dm-exception-store.h
@@ -75,6 +75,23 @@ struct dm_exception_store_type {
 				  void *callback_context);
 
 	/*
+	 * Returns 0 if the exception store is empty.
+	 *
+	 * If there are exceptions still to be merged, sets
+	 * *last_old_chunk and *last_new_chunk to the most recent
+	 * still-to-be-merged chunk and returns the number of
+	 * consecutive previous ones.
+	 */
+	int (*prepare_merge) (struct dm_exception_store *store,
+			      chunk_t *last_old_chunk, chunk_t *last_new_chunk);
+
+	/*
+	 * Clear the last n exceptions.
+	 * nr_merged must be <= the value returned by prepare_merge.
+	 */
+	int (*commit_merge) (struct dm_exception_store *store, int nr_merged);
+
+	/*
 	 * The snapshot is invalid, note this in the metadata.
 	 */
 	void (*drop_snapshot) (struct dm_exception_store *store);