dm snapshot: use dm-bufio prefetch

This patch modifies dm-snapshot so that it prefetches the buffers when
loading the exceptions.

The number of buffers read ahead is specified in the DM_PREFETCH_CHUNKS
macro.  The current value for DM_PREFETCH_CHUNKS (12) was found to
provide the best performance on a single 15k SCSI spindle.  In the
future we may modify this default or make it configurable.

Also, introduce the function dm_bufio_set_minimum_buffers to setup
bufio's number of internal buffers before freeing happens.  dm-bufio may
hold more buffers if enough memory is available.  There is no guarantee
that the specified number of buffers will be available - if you need a
guarantee, use the argument reserved_buffers for
dm_bufio_client_create.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

1 files changed, 24 insertions, 1 deletions

diff --git a/drivers/md/dm-snap-persistent.c b/drivers/md/dm-snap-persistent.c
index 169275050c0b..afc3d017de4c 100644
--- a/drivers/md/dm-snap-persistent.c
+++ b/drivers/md/dm-snap-persistent.c
@@ -18,6 +18,8 @@
 #define DM_MSG_PREFIX "persistent snapshot"
 #define DM_CHUNK_SIZE_DEFAULT_SECTORS 32        /* 16KB */
 
+#define DM_PREFETCH_CHUNKS              12
+
 /*-----------------------------------------------------------------
  * Persistent snapshots, by persistent we mean that the snapshot
  * will survive a reboot.
@@ -497,6 +499,7 @@ static int read_exceptions(struct pstore *ps,
 {
         int r, full = 1;
         struct dm_bufio_client *client;
+        chunk_t prefetch_area = 0;
 
         client = dm_bufio_client_create(dm_snap_cow(ps->store->snap)->bdev,
                                         ps->store->chunk_size << SECTOR_SHIFT,
@@ -506,13 +509,33 @@ static int read_exceptions(struct pstore *ps,
                 return PTR_ERR(client);
 
         /*
+         * Setup for one current buffer + desired readahead buffers.
+         */
+        dm_bufio_set_minimum_buffers(client, 1 + DM_PREFETCH_CHUNKS);
+
+        /*
          * Keeping reading chunks and inserting exceptions until
          * we find a partially full area.
          */
         for (ps->current_area = 0; full; ps->current_area++) {
                 struct dm_buffer *bp;
                 void *area;
-                chunk_t chunk = area_location(ps, ps->current_area);
+                chunk_t chunk;
+
+                if (unlikely(prefetch_area < ps->current_area))
+                        prefetch_area = ps->current_area;
+
+                if (DM_PREFETCH_CHUNKS) do {
+                        chunk_t pf_chunk = area_location(ps, prefetch_area);
+                        if (unlikely(pf_chunk >= dm_bufio_get_device_size(client)))
+                                break;
+                        dm_bufio_prefetch(client, pf_chunk, 1);
+                        prefetch_area++;
+                        if (unlikely(!prefetch_area))
+                                break;
+                } while (prefetch_area <= ps->current_area + DM_PREFETCH_CHUNKS);
+
+                chunk = area_location(ps, ps->current_area);
 
                 area = dm_bufio_read(client, chunk, &bp);
                 if (unlikely(IS_ERR(area))) {