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>
diff --git a/drivers/md/dm-bufio.c b/drivers/md/dm-bufio.c
index d865937..9ed4212 100644
--- a/drivers/md/dm-bufio.c
+++ b/drivers/md/dm-bufio.c
@@ -104,6 +104,8 @@
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 @@
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 @@
}
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 @@
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;
diff --git a/drivers/md/dm-bufio.h b/drivers/md/dm-bufio.h
index 3dac376..c096779 100644
--- a/drivers/md/dm-bufio.h
+++ b/drivers/md/dm-bufio.h
@@ -115,6 +115,11 @@
*/
void dm_bufio_forget(struct dm_bufio_client *c, sector_t block);
+/*
+ * Set the minimum number of buffers before cleanup happens.
+ */
+void dm_bufio_set_minimum_buffers(struct dm_bufio_client *c, unsigned n);
+
unsigned dm_bufio_get_block_size(struct dm_bufio_client *c);
sector_t dm_bufio_get_device_size(struct dm_bufio_client *c);
sector_t dm_bufio_get_block_number(struct dm_buffer *b);
diff --git a/drivers/md/dm-snap-persistent.c b/drivers/md/dm-snap-persistent.c
index 1692750..afc3d01 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 @@
{
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 @@
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))) {