md: expose max value of behind writes counter

Keep track of the maximum number of concurrent write-behind requests
for an md array and exposed this number in sysfs at
   md/bitmap/max_backlog_used

Writing any value to this file will clear it.

This allows userspace to be involved in tuning bitmap/backlog.

Signed-off-by: Paul Clements <paul.clements@steeleye.com>
Signed-off-by: NeilBrown <neilb@suse.de>

1 files changed, 1 insertions, 0 deletions

diff --git a/drivers/md/bitmap.h b/drivers/md/bitmap.h
index cb821d76d1b4..aa82b7caa85f 100644
--- a/drivers/md/bitmap.h
+++ b/drivers/md/bitmap.h
@@ -227,6 +227,7 @@ struct bitmap {
         int allclean;
 
         atomic_t behind_writes;
+        unsigned long behind_writes_used; /* highest actual value at runtime */
 
         /*
          * the bitmap daemon - periodically wakes up and sweeps the bitmap