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>

2 files changed, 19 insertions, 4 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.
diff --git a/fs/ext4/super.c b/fs/ext4/super.c
index ba92c606ad9a..cb96f127c366 100644
--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -671,6 +671,7 @@ static int ext4_show_options(struct seq_file *seq, struct vfsmount *vfs)
         unsigned long def_mount_opts;
         struct super_block *sb = vfs->mnt_sb;
         struct ext4_sb_info *sbi = EXT4_SB(sb);
+        journal_t *journal = sbi->s_journal;
         struct ext4_super_block *es = sbi->s_es;
 
         def_mount_opts = le32_to_cpu(es->s_default_mount_opts);
@@ -729,8 +730,13 @@ static int ext4_show_options(struct seq_file *seq, struct vfsmount *vfs)
                 seq_printf(seq, ",commit=%u",
                            (unsigned) (sbi->s_commit_interval / HZ));
         }
-        if (test_opt(sb, BARRIER))
-                seq_puts(seq, ",barrier=1");
+        /*
+         * We're changing the default of barrier mount option, so
+         * let's always display its mount state so it's clear what its
+         * status is.
+         */
+        seq_puts(seq, ",barrier=");
+        seq_puts(seq, test_opt(sb, BARRIER) ? "1" : "0");
         if (test_opt(sb, JOURNAL_ASYNC_COMMIT))
                 seq_puts(seq, ",journal_async_commit");
         if (test_opt(sb, NOBH))
@@ -1909,6 +1915,7 @@ static int ext4_fill_super (struct super_block *sb, void *data, int silent)
         sbi->s_resgid = le16_to_cpu(es->s_def_resgid);
 
         set_opt(sbi->s_mount_opt, RESERVATION);
+        set_opt(sbi->s_mount_opt, BARRIER);
 
         /*
          * turn on extents feature by default in ext4 filesystem