ext4: Regularize mount options

Add support for using the mount options "barrier" and "nobarrier", and
"auto_da_alloc" and "noauto_da_alloc", which is more consistent than
"barrier=<0|1>" or "auto_da_alloc=<0|1>".  Most other ext3/ext4 mount
options use the foo/nofoo naming convention.  We allow the old forms
of these mount options for backwards compatibility.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

1 files changed, 23 insertions, 2 deletions

diff --git a/Documentation/filesystems/ext4.txt b/Documentation/filesystems/ext4.txt
index 5c484aec2ba..97882df0486 100644
--- a/Documentation/filesystems/ext4.txt
+++ b/Documentation/filesystems/ext4.txt
@@ -183,8 +183,8 @@ commit=nrsec	(*)	Ext4 can be told to sync all its data and metadata
                         performance.
 
 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
+barrier(*)              the jbd code.  barrier=0 disables, barrier=1 enables.
+nobarrier               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
@@ -192,6 +192,9 @@ barrier=<0|1(*)>	This enables/disables the use of write barriers in
                         safe to use, at some performance penalty.  If
                         your disks are battery-backed in one way or another,
                         disabling barriers may safely improve performance.
+                        The mount options "barrier" and "nobarrier" can
+                        also be used to enable or disable barriers, for
+                        consistency with other ext4 mount options.
 
 inode_readahead=n       This tuning parameter controls the maximum
                         number of inode table blocks that ext4's inode
@@ -313,6 +316,24 @@ journal_ioprio=prio	The I/O priority (from 0 to 7, where 0 is the
                         a slightly higher priority than the default I/O
                         priority.
 
+auto_da_alloc(*)        Many broken applications don't use fsync() when 
+noauto_da_alloc         replacing existing files via patterns such as
+                        fd = open("foo.new")/write(fd,..)/close(fd)/
+                        rename("foo.new", "foo"), or worse yet,
+                        fd = open("foo", O_TRUNC)/write(fd,..)/close(fd).
+                        If auto_da_alloc is enabled, ext4 will detect
+                        the replace-via-rename and replace-via-truncate
+                        patterns and force that any delayed allocation
+                        blocks are allocated such that at the next
+                        journal commit, in the default data=ordered
+                        mode, the data blocks of the new file are forced
+                        to disk before the rename() operation is
+                        commited.  This provides roughly the same level
+                        of guarantees as ext3, and avoids the
+                        "zero-length" problem that can happen when a
+                        system crashes before the delayed allocation
+                        blocks are forced to disk.
+
 Data Mode
 =========
 There are 3 different data modes: