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    SubjectRe: Device loses barrier support (was: Fixed patch for simple barriers.)


    On Fri, 5 Dec 2008, Andi Kleen wrote:

    > > * barrier support in md-raid1 deviates from the specification at
    > > Documentation/block/barrier.txt. The specification says that requests
    > > submitted after the barrier request hit the media after the barrier
    > > request hits the media. The reality is that the barrier request can be
    > > randomly aborted and the requests submitted after it hit the media before
    > > the barrier request.
    >
    > Yes the spec should be probably updated.
    >
    > But also see Linus' rant from yesterday about code vs documentation.
    > When in doubt the code wins.

    The only one offender is "md". It is less overhead to change "md" to play
    nice and be reliable than to double-submit requests in all the places that
    needs write ordering.

    > > * the filesystems developed hacks to work around this issue, the hacks
    > > involve not submitting more requests after the barrier request,
    >
    > I suspect the reason the file systems did it this way is that
    > it was a much simpler change than to rewrite the transaction
    > manager for this.

    It could be initial reason. But this unreliability also disallows any
    improvement in filesystems. No one can write asynchronous transaction
    manager because of that evil EOPNOTSUPP.

    > > synchronously waiting for the barrier request and eventually retrying it.
    > > These hacks suppress any performance advantage barriers could bring.
    > >
    > > * you submit a patch that makes barriers even more often deviate from the
    > > specification and you argue that the patch is correct because filesystems
    > > handle this deviation.
    >
    > Sorry what counts is the code behaviour, not the specification.

    Better interface is that one that has less maintenance overhead. And I
    don't see requiring the programmers of all IO code to double-submit
    requests as less maintenance overhead.

    > -Andi

    Mikulas

    ---

    If you want to make it easier to infer functionality from the code, apply
    this patch :)

    ---
    block/blk-core.c | 8 ++++++++
    1 file changed, 8 insertions(+)

    Index: linux-2.6.28-rc5-devel/block/blk-core.c
    ===================================================================
    --- linux-2.6.28-rc5-devel.orig/block/blk-core.c 2008-12-05 02:54:25.000000000 +0100
    +++ linux-2.6.28-rc5-devel/block/blk-core.c 2008-12-05 03:14:23.000000000 +0100
    @@ -28,6 +28,7 @@
    #include <linux/task_io_accounting_ops.h>
    #include <linux/blktrace_api.h>
    #include <linux/fault-inject.h>
    +#include <linux/random.h>

    #include "blk.h"

    @@ -1528,6 +1529,13 @@ void submit_bio(int rw, struct bio *bio)

    bio->bi_rw |= rw;

    + /* At least, make the true nature of write barriers obvious. */
    +
    + if (bio_barrier(bio) && !(random32() % 42)) {
    + bio_endio(bio, -EOPNOTSUPP);
    + return;
    + }
    +
    /*
    * If it's a regular read/write or a barrier with data attached,
    * go through the normal accounting stuff before submission.

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      Last update: 2008-12-05 03:29    [W:0.028 / U:0.484 seconds]
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