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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 <>
    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

    -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

     \ /
      Last update: 2008-05-16 21:13    [W:0.022 / U:31.032 seconds]
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