block: add iostat counters for flush requests

Requests that triggers flushing volatile writeback cache to disk (barriers)
have significant effect to overall performance.

Block layer has sophisticated engine for combining several flush requests
into one. But there is no statistics for actual flushes executed by disk.
Requests which trigger flushes usually are barriers - zero-size writes.

This patch adds two iostat counters into /sys/class/block/$dev/stat and
/proc/diskstats - count of completed flush requests and their total time.

Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

1 files changed, 1 insertions, 0 deletions

diff --git a/include/linux/blk_types.h b/include/linux/blk_types.h
index 23a2fd534817..70254ae11769 100644
--- a/include/linux/blk_types.h
+++ b/include/linux/blk_types.h
@@ -377,6 +377,7 @@ enum stat_group {
 	STAT_READ,
 	STAT_WRITE,
 	STAT_DISCARD,
+	STAT_FLUSH,
 
 	NR_STAT_GROUPS
 };