ext4: add fsync batch tuning knobs

[~andy/linux] / Documentation / filesystems / ext4.txt

diff --git a/Documentation/filesystems/ext4.txt b/Documentation/filesystems/ext4.txt
index f75ab101c00af95557d112286c2fea8f880e7ccf..e3fcbea3ec8c2b2ded706407c2c5f08264467d14 100644 (file)
--- a/Documentation/filesystems/ext4.txt
+++ b/Documentation/filesystems/ext4.txt
@@ -283,6 +283,35 @@ delalloc   (*)     Deferring block allocation until write-out time.
 nodelalloc             Disable delayed allocation. Blocks are allocation
                        when data is copied from user to page cache.
 
+max_batch_time=usec    Maximum amount of time ext4 should wait for
+                       additional filesystem operations to be batch
+                       together with a synchronous write operation.
+                       Since a synchronous write operation is going to
+                       force a commit and then a wait for the I/O
+                       complete, it doesn't cost much, and can be a
+                       huge throughput win, we wait for a small amount
+                       of time to see if any other transactions can
+                       piggyback on the synchronous write.   The
+                       algorithm used is designed to automatically tune
+                       for the speed of the disk, by measuring the
+                       amount of time (on average) that it takes to
+                       finish committing a transaction.  Call this time
+                       the "commit time".  If the time that the
+                       transactoin has been running is less than the
+                       commit time, ext4 will try sleeping for the
+                       commit time to see if other operations will join
+                       the transaction.   The commit time is capped by
+                       the max_batch_time, which defaults to 15000us
+                       (15ms).   This optimization can be turned off
+                       entirely by setting max_batch_time to 0.
+
+min_batch_time=usec    This parameter sets the commit time (as
+                       described above) to be at least min_batch_time.
+                       It defaults to zero microseconds.  Increasing
+                       this parameter may improve the throughput of
+                       multi-threaded, synchronous workloads on very
+                       fast disks, at the cost of increasing latency.
+
 Data Mode
 =========
 There are 3 different data modes: