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, 12 insertions, 2 deletions

diff --git a/drivers/md/dm-bufio.c b/drivers/md/dm-bufio.c
index d86593721915..9ed42125514b 100644
--- a/drivers/md/dm-bufio.c
+++ b/drivers/md/dm-bufio.c
@@ -104,6 +104,8 @@ struct dm_bufio_client {
         struct list_head reserved_buffers;
         unsigned need_reserved_buffers;
 
+        unsigned minimum_buffers;
+
         struct hlist_head *cache_hash;
         wait_queue_head_t free_buffer_wait;
 
@@ -861,8 +863,8 @@ static void __get_memory_limit(struct dm_bufio_client *c,
         buffers = dm_bufio_cache_size_per_client >>
                   (c->sectors_per_block_bits + SECTOR_SHIFT);
 
-        if (buffers < DM_BUFIO_MIN_BUFFERS)
-                buffers = DM_BUFIO_MIN_BUFFERS;
+        if (buffers < c->minimum_buffers)
+                buffers = c->minimum_buffers;
 
         *limit_buffers = buffers;
         *threshold_buffers = buffers * DM_BUFIO_WRITEBACK_PERCENT / 100;
@@ -1372,6 +1374,12 @@ void dm_bufio_forget(struct dm_bufio_client *c, sector_t block)
 }
 EXPORT_SYMBOL(dm_bufio_forget);
 
+void dm_bufio_set_minimum_buffers(struct dm_bufio_client *c, unsigned n)
+{
+        c->minimum_buffers = n;
+}
+EXPORT_SYMBOL(dm_bufio_set_minimum_buffers);
+
 unsigned dm_bufio_get_block_size(struct dm_bufio_client *c)
 {
         return c->block_size;
@@ -1568,6 +1576,8 @@ struct dm_bufio_client *dm_bufio_client_create(struct block_device *bdev, unsign
         INIT_LIST_HEAD(&c->reserved_buffers);
         c->need_reserved_buffers = reserved_buffers;
 
+        c->minimum_buffers = DM_BUFIO_MIN_BUFFERS;
+
         init_waitqueue_head(&c->free_buffer_wait);
         c->async_write_error = 0;