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Message-ID: <482CA094.20703@redhat.com>
Date: Thu, 15 May 2008 15:44:04 -0500
From: Eric Sandeen <sandeen@...hat.com>
To: Jan Kara <jack@...e.cz>
CC: ext4 development <linux-ext4@...r.kernel.org>
Subject: Re: barriers off by default?
Jan Kara wrote:
>> As I look at my shiny new 500G disks with 32MB of cache, I find myself
>> wondering why the default for ext3 and ext4 is to have barriers disabled.
>>
>> This is a pretty dangerous default w.r.t. filesystem integrity on power
>> loss, no?
>>
> JFYI: SUSE kernel carries for ages a patch which changes this default.
> I'd be more than happy to drop it ;).
>
> Honza
>
What do folks think of this?
the show_options change is a little funky since jbd may do a test
write and fail... (actually I was thinking maybe at fill_super we
should do a test barrier write and get it out of the way early...)
-Eric
diff --git a/Documentation/filesystems/ext3.txt b/Documentation/filesystems/ext3.txt
index b45f3c1..daab1f5 100644
--- a/Documentation/filesystems/ext3.txt
+++ b/Documentation/filesystems/ext3.txt
@@ -52,8 +52,16 @@ commit=nrsec (*) Ext3 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/ext3/super.c b/fs/ext3/super.c
index fe3119a..d06e0f3 100644
--- a/fs/ext3/super.c
+++ b/fs/ext3/super.c
@@ -555,6 +555,7 @@ static int ext3_show_options(struct seq_file *seq, struct vfsmount *vfs)
struct super_block *sb = vfs->mnt_sb;
struct ext3_sb_info *sbi = EXT3_SB(sb);
struct ext3_super_block *es = sbi->s_es;
+ journal_t *journal = EXT3_SB(sb)->s_journal;
unsigned long def_mount_opts;
def_mount_opts = le32_to_cpu(es->s_default_mount_opts);
@@ -613,8 +614,16 @@ static int ext3_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");
+ if (!test_opt(sb, BARRIER)) {
+ seq_puts(seq, ",barrier=0");
+ } else {
+ /*
+ * jbd inherits the barrier flag from ext3, and jbd may actually
+ * turn off barriers if a write fails, so it's the real test.
+ */
+ if (journal && !(journal->j_flags & JFS_BARRIER))
+ seq_puts(seq, ",barrier=1(failed)");
+ }
if (test_opt(sb, NOBH))
seq_puts(seq, ",nobh");
@@ -1589,6 +1598,7 @@ static int ext3_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);
if (!parse_options ((char *) data, sb, &journal_inum, &journal_devnum,
NULL, 0))
--
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