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dm snapshot: use dm-bufio prefetch

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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>
This commit is contained in:
Mikulas Patocka
2014-01-13 19:13:05 -05:00
committed by Mike Snitzer
parent 55494bf294
commit 55b082e614
3 changed files with 41 additions and 3 deletions
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25
drivers/md/dm-snap-persistent.c
View File

@@ -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,
@@ -505,6 +508,11 @@ static int read_exceptions(struct pstore *ps,
if (IS_ERR(client))
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.
@@ -512,7 +520,22 @@ 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;
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))) {