[PATCH v2 7/7] dm snapshot: use bufio prefetch

[Mike Snitzer <snitzer@redhat.com>]

From: Mikulas Patocka <mpatocka@redhat.com>

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

The number of chunk-sized buffers read ahead is specified in the
DM_PREFETCH_CHUNKS macro.  The current default for DM_PREFETCH_CHUNKS
(12) may need to be adjusted to improve performance on different types
of storage -- a future patch should make this configurable.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
---
 drivers/md/dm-snap-persistent.c |   34 +++++++++++++++++++++++++++++++++-
 1 files changed, 33 insertions(+), 1 deletions(-)

diff --git a/drivers/md/dm-snap-persistent.c b/drivers/md/dm-snap-persistent.c
index 1692750..ea45b3b 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.
@@ -490,6 +492,30 @@ static int insert_exceptions(struct pstore *ps, void *ps_area,
 	return 0;
 }
 
+static void bufio_prefetch_chunks(struct dm_bufio_client *client,
+				  struct pstore *ps)
+{
+	chunk_t prefetch_area = 0;
+	chunk_t pf_chunk;
+
+	if (!DM_PREFETCH_CHUNKS)
+		return;
+
+	if (prefetch_area < ps->current_area)
+		prefetch_area = ps->current_area;
+
+	do {
+		pf_chunk = area_location(ps, prefetch_area);
+		if (unlikely(pf_chunk >= dm_bufio_get_device_size(client)))
+			break;
+		if (unlikely(!dm_bufio_prefetch(client, pf_chunk, 1)))
+			break;
+		prefetch_area++;
+		if (unlikely(!prefetch_area))
+			break;
+	} while (prefetch_area <= ps->current_area + DM_PREFETCH_CHUNKS);
+}
+
 static int read_exceptions(struct pstore *ps,
 			   int (*callback)(void *callback_context, chunk_t old,
 					   chunk_t new),
@@ -505,6 +531,8 @@ static int read_exceptions(struct pstore *ps,
 	if (IS_ERR(client))
 		return PTR_ERR(client);
 
+	dm_bufio_set_minimum_buffers(client, DM_PREFETCH_CHUNKS + 1);
+
 	/*
 	 * Keeping reading chunks and inserting exceptions until
 	 * we find a partially full area.
@@ -512,7 +540,11 @@ static int read_exceptions(struct pstore *ps,
 	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;
+
+		bufio_prefetch_chunks(client, ps);
+
+		chunk = area_location(ps, ps->current_area);
 
 		area = dm_bufio_read(client, chunk, &bp);
 		if (unlikely(IS_ERR(area))) {
-- 
1.7.4.4