Re: [ext4] Documentation patch

[Theodore Tso <tytso@mit.edu>]

This is what I have added to the ext4 patch queue.

							- Ted

Update Documentation/filesystems/ext4.txt

Fix paragraph with recommendations on how to tune ext4 for benchmarks.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
diff --git a/Documentation/filesystems/ext4.txt b/Documentation/filesystems/ext4.txt
index 845e691..19bb93f 100644
--- a/Documentation/filesystems/ext4.txt
+++ b/Documentation/filesystems/ext4.txt
@@ -58,13 +58,18 @@ Note: More extensive information for getting started with ext4 can be
 
 	# mount -t ext4 /dev/hda1 /wherever
 
-  - When comparing performance with other filesystems, remember that
-    ext3/4 by default offers higher data integrity guarantees than most.
-    So when comparing with a metadata-only journalling filesystem, such
-    as ext3, use `mount -o data=writeback'.  And you might as well use
-    `mount -o nobh' too along with it.  Making the journal larger than
-    the mke2fs default often helps performance with metadata-intensive
-    workloads.
+  - When comparing performance with other filesystems, it's always
+    important to try multiple workloads; very often a subtle change in a
+    workload parameter can completely change the ranking of which
+    filesystems do well compared to others.  When comparing versus ext3,
+    note that ext4 enables write barriers by default, while ext3 does
+    not enable write barriers by default.  So it is useful to use
+    explicitly specify whether barriers are enabled or not when via the
+    '-o barriers=[0|1]' mount option.  When tuning ext3 for best
+    benchmark numbers, it is often worthwhile to try changing the data
+    journaling mode; '-o data=writeback,nobh' can be faster for some
+    workloads.  A large journal can also be helpful for
+    metadata-intensive workloads.
 
 2. Features
 ===========