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Subject[PATCH 3/4] 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>
---
Documentation/filesystems/ext4.txt | 12 ++++++++++--
fs/ext4/super.c | 11 +++++++++--
2 files changed, 19 insertions(+), 4 deletions(-)
diff --git a/Documentation/filesystems/ext4.txt b/Documentation/filesystems/ext4.txt
index 560f88d..0c5086d 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 52dd067..77b036a 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");
+ /*
+ * jbd2 inherits the barrier flag from ext4, and may actually
+ * turn off barriers if a write fails, so it's the real test.
+ */
+ if (!test_opt(sb, BARRIER) ||
+ (journal && !(journal->j_flags & JBD2_BARRIER)))
+ seq_puts(seq, ",barrier=0");
if (test_opt(sb, NOBH))
seq_puts(seq, ",nobh");
if (!test_opt(sb, EXTENTS))
@@ -1890,6 +1896,7 @@ 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
-- 1.5.3.6


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