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SubjectRe: Mount ext3 with barrier=1 doesn't send real barrier bio?
Milan Broz wrote:
> Hi,
>
> I run some barrier tests over device-mapper (which currently doesn't
> support barrier bio at all) and even if I set barrier=1 in ext3 mount,
> there is never any bio with barrier flag... (in 2.6.27-rc)
>
> How is the barrier=1 flag supposed to work in ext3 (JBD) now?

Milan, you're right. Ric saw this same strange behavior when doing some
benchmarking with and without barriers; Chris noticed the change in
submit_bh; I was about to write up a similar patch to what you've sent
already. Jens, does Milan's fix look good to you?

Incidentally, I ran Ric's test on ext3 on a sata drive:

# fs_mark -d /mnt/test -n 1600 -t 1 -s 20480

cfq:
files/s
2.6.25 2.6.26.2 2.6.26.2+patch
barrier=0 169 127 126
barrier=1 33 126 33

noop:
files/s
2.6.25 2.6.26.2 2.6.26.2+patch
barrier=0 191 184 185
barrier=1 33 180 33

deadline:
files/s
2.6.25 2.6.26.2 2.6.26.2+patch
barrier=0 181 182 185
barrier=1 33 185 33

anticipatory:
files/s
2.6.25 2.6.26.2 2.6.26.2+patch
barrier=0 187 133 132
barrier=1 34 134 33

-Eric

> See:
> If you specify barrier=1, JFS_BARRIER flag is set in ext3_init_journal_params
> journal->j_flags |= JFS_BARRIER;
>
> Now, journal_write_commit_record is called and this happens:
>
> if (journal->j_flags & JFS_BARRIER) {
> set_buffer_ordered(bh);
> barrier_done = 1;
> }
> ret = sync_dirty_buffer(bh);
>
> if (barrier_done)
> clear_buffer_ordered(bh);
>
> if (ret == -EOPNOTSUPP && barrier_done) {
> ...
>
> From this code I expect that EOPNOTSUPP is returned if barrier is not
> supported (yes, that exactly does device-mapper now without barrier patches).
>
> But it *never* happens because:
>
> sync_dirty_buffer always calls
> submit_bh(WRITE_SYNC, bh)
>
> and in submit_bh is this test:
>
> if (buffer_ordered(bh) && (rw == WRITE))
> rw = WRITE_BARRIER;
>
> but there is rw == WRITE_SYNC, not WRITE !
>
> So the barrier flag for bio is never set and normal sync write
> is performed.
>
> Why it isn't done like in attached patch? Is it intentional or it is bug?
>
> I think it was caused by change in this commit:
>
> commit 18ce3751ccd488c78d3827e9f6bf54e6322676fb
> Author: Jens Axboe <jens.axboe@oracle.com>
> Date: Tue Jul 1 09:07:34 2008 +0200
>
> Properly notify block layer of sync writes
>
> Milan
> --
>
> Set BIO_RW_BARRIER flag even for submit_bh sync write request.
>
> Signed-off-by: Milan Broz <mbroz@redhat.com>
> ---
> fs/buffer.c | 8 ++++----
> 1 files changed, 4 insertions(+), 4 deletions(-)
>
> --- a/fs/buffer.c
> +++ b/fs/buffer.c
> @@ -2926,16 +2926,16 @@ int submit_bh(int rw, struct buffer_head * bh)
> BUG_ON(!buffer_mapped(bh));
> BUG_ON(!bh->b_end_io);
>
> - if (buffer_ordered(bh) && (rw == WRITE))
> - rw = WRITE_BARRIER;
> -
> /*
> * Only clear out a write error when rewriting, should this
> * include WRITE_SYNC as well?
> */
> - if (test_set_buffer_req(bh) && (rw == WRITE || rw == WRITE_BARRIER))
> + if (test_set_buffer_req(bh) && rw == WRITE)
> clear_buffer_write_io_error(bh);
>
> + if (buffer_ordered(bh) && ((rw & RW_MASK) == WRITE))
> + rw |= (1 << BIO_RW_BARRIER);
> +
> /*
> * from here on down, it's all bio -- do the initial mapping,
> * submit_bio -> generic_make_request may further map this bio around
>
>
>
>
> --
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