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>

diff --git a/drivers/md/dm-exception-store.h b/drivers/md/dm-exception-store.h
index bb88746..c53e089 100644 (file)
--- a/drivers/md/dm-exception-store.h
+++ b/drivers/md/dm-exception-store.h
@@ -74,6 +74,23 @@ struct dm_exception_store_type {
                                  void (*callback) (void *, int success),
                                  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.
         */

diff --git a/drivers/md/dm-snap-persistent.c b/drivers/md/dm-snap-persistent.c
index 157999e..7d08879 100644 (file)
--- a/drivers/md/dm-snap-persistent.c
+++ b/drivers/md/dm-snap-persistent.c
@@ -55,6 +55,8 @@
  */
 #define SNAPSHOT_DISK_VERSION 1
 
+#define NUM_SNAPSHOT_HDR_CHUNKS 1
+
 struct disk_header {
        uint32_t magic;
 
@@ -120,7 +122,22 @@ struct pstore {
 
        /*
         * The next free chunk for an exception.
+        *
+        * When creating exceptions, all the chunks here and above are
+        * free.  It holds the next chunk to be allocated.  On rare
+        * occasions (e.g. after a system crash) holes can be left in
+        * the exception store because chunks can be committed out of
+        * order.
+        *
+        * When merging exceptions, it does not necessarily mean all the
+        * chunks here and above are free.  It holds the value it would
+        * have held if all chunks had been committed in order of
+        * allocation.  Consequently the value may occasionally be
+        * slightly too low, but since it's only used for 'status' and
+        * it can never reach its minimum value too early this doesn't
+        * matter.
         */
+
        chunk_t next_free;
 
        /*
@@ -409,6 +426,15 @@ static void write_exception(struct pstore *ps,
        e->new_chunk = cpu_to_le64(de->new_chunk);
 }
 
+static void clear_exception(struct pstore *ps, uint32_t index)
+{
+       struct disk_exception *e = get_exception(ps, index);
+
+       /* clear it */
+       e->old_chunk = 0;
+       e->new_chunk = 0;
+}
+
 /*
  * Registers the exceptions that are present in the current area.
  * 'full' is filled in to indicate if the area has been
@@ -505,7 +531,8 @@ static void persistent_usage(struct dm_exception_store *store,
         * Then there are (ps->current_area + 1) metadata chunks, each one
         * separated from the next by ps->exceptions_per_area data chunks.
         */
-       *metadata_sectors = (ps->current_area + 2) * store->chunk_size;
+       *metadata_sectors = (ps->current_area + 1 + NUM_SNAPSHOT_HDR_CHUNKS) *
+                           store->chunk_size;
 }
 
 static void persistent_dtr(struct dm_exception_store *store)
@@ -680,6 +707,85 @@ static void persistent_commit_exception(struct dm_exception_store *store,
        ps->callback_count = 0;
 }
 
+static int persistent_prepare_merge(struct dm_exception_store *store,
+                                   chunk_t *last_old_chunk,
+                                   chunk_t *last_new_chunk)
+{
+       struct pstore *ps = get_info(store);
+       struct disk_exception de;
+       int nr_consecutive;
+       int r;
+
+       /*
+        * When current area is empty, move back to preceding area.
+        */
+       if (!ps->current_committed) {
+               /*
+                * Have we finished?
+                */
+               if (!ps->current_area)
+                       return 0;
+
+               ps->current_area--;
+               r = area_io(ps, READ);
+               if (r < 0)
+                       return r;
+               ps->current_committed = ps->exceptions_per_area;
+       }
+
+       read_exception(ps, ps->current_committed - 1, &de);
+       *last_old_chunk = de.old_chunk;
+       *last_new_chunk = de.new_chunk;
+
+       /*
+        * Find number of consecutive chunks within the current area,
+        * working backwards.
+        */
+       for (nr_consecutive = 1; nr_consecutive < ps->current_committed;
+            nr_consecutive++) {
+               read_exception(ps, ps->current_committed - 1 - nr_consecutive,
+                              &de);
+               if (de.old_chunk != *last_old_chunk - nr_consecutive ||
+                   de.new_chunk != *last_new_chunk - nr_consecutive)
+                       break;
+       }
+
+       return nr_consecutive;
+}
+
+static int persistent_commit_merge(struct dm_exception_store *store,
+                                  int nr_merged)
+{
+       int r, i;
+       struct pstore *ps = get_info(store);
+
+       BUG_ON(nr_merged > ps->current_committed);
+
+       for (i = 0; i < nr_merged; i++)
+               clear_exception(ps, ps->current_committed - 1 - i);
+
+       r = area_io(ps, WRITE);
+       if (r < 0)
+               return r;
+
+       ps->current_committed -= nr_merged;
+
+       /*
+        * At this stage, only persistent_usage() uses ps->next_free, so
+        * we make no attempt to keep ps->next_free strictly accurate
+        * as exceptions may have been committed out-of-order originally.
+        * Once a snapshot has become merging, we set it to the value it
+        * would have held had all the exceptions been committed in order.
+        *
+        * ps->current_area does not get reduced by prepare_merge() until
+        * after commit_merge() has removed the nr_merged previous exceptions.
+        */
+       ps->next_free = (area_location(ps, ps->current_area) - 1) +
+                       (ps->current_committed + 1) + NUM_SNAPSHOT_HDR_CHUNKS;
+
+       return 0;
+}
+
 static void persistent_drop_snapshot(struct dm_exception_store *store)
 {
        struct pstore *ps = get_info(store);
@@ -705,7 +811,7 @@ static int persistent_ctr(struct dm_exception_store *store,
        ps->area = NULL;
        ps->zero_area = NULL;
        ps->header_area = NULL;
-       ps->next_free = 2;      /* skipping the header and first area */
+       ps->next_free = NUM_SNAPSHOT_HDR_CHUNKS + 1; /* header and 1st area */
        ps->current_committed = 0;
 
        ps->callback_count = 0;
@@ -748,6 +854,8 @@ static struct dm_exception_store_type _persistent_type = {
        .read_metadata = persistent_read_metadata,
        .prepare_exception = persistent_prepare_exception,
        .commit_exception = persistent_commit_exception,
+       .prepare_merge = persistent_prepare_merge,
+       .commit_merge = persistent_commit_merge,
        .drop_snapshot = persistent_drop_snapshot,
        .usage = persistent_usage,
        .status = persistent_status,
@@ -761,6 +869,8 @@ static struct dm_exception_store_type _persistent_compat_type = {
        .read_metadata = persistent_read_metadata,
        .prepare_exception = persistent_prepare_exception,
        .commit_exception = persistent_commit_exception,
+       .prepare_merge = persistent_prepare_merge,
+       .commit_merge = persistent_commit_merge,
        .drop_snapshot = persistent_drop_snapshot,
        .usage = persistent_usage,
        .status = persistent_status,