ext4: enable barriers by default

I can't think of any valid reason for ext4 to not use barriers when
they are available;  I believe this is necessary for filesystem
integrity in the face of a volatile write cache on storage.

An administrator who trusts that the cache is sufficiently battery-
backed (and power supplies are sufficiently redundant, etc...)
can always turn it back off again.

SuSE has carried such a patch for ext3 for quite some time now.

Also document the mount option while we're at it.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Mingming Cao <cmm@us.ibm.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

1 files changed, 10 insertions, 2 deletions

diff --git a/Documentation/filesystems/ext4.txt b/Documentation/filesystems/ext4.txt
index 560f88dc7090..0c5086db8352 100644
--- a/Documentation/filesystems/ext4.txt
+++ b/Documentation/filesystems/ext4.txt
@@ -139,8 +139,16 @@ commit=nrsec	(*)	Ext4 can be told to sync all its data and metadata
                         Setting it to very large values will improve
                         performance.
 
-barrier=1               This enables/disables barriers.  barrier=0 disables
-                        it, barrier=1 enables it.
+barrier=<0|1(*)>        This enables/disables the use of write barriers in
+                        the jbd code.  barrier=0 disables, barrier=1 enables.
+                        This also requires an IO stack which can support
+                        barriers, and if jbd gets an error on a barrier
+                        write, it will disable again with a warning.
+                        Write barriers enforce proper on-disk ordering
+                        of journal commits, making volatile disk write caches
+                        safe to use, at some performance penalty.  If
+                        your disks are battery-backed in one way or another,
+                        disabling barriers may safely improve performance.
 
 orlov           (*)     This enables the new Orlov block allocator. It is
                         enabled by default.