question about BIO/request ordering / barriers

From: Christophe Saout (christophe_at_saout.de)
Date: 12/31/03

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    To: Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Date:	Wed, 31 Dec 2003 15:01:07 +0100

    Hi!

    I'm just digging through the device-mapper code and a question came up:

    Are "intermediate block device drivers" (like device-mapper) allowed to
    reorder BIOs?

    I'm not talking about BIOs submitted from different threads at the same
    time but BIOs submitted from the same thread sequentially, especially
    writes.

    That would mean that BIOs might be reordered around barriers which would
    break potential users.

    At the moment I suppose this shouldn't be an issue because I didn't find
    a single user in the whole kernel that actually submits BIOs with
    BIO_RW_BARRIER set via submit_bio/generic_make_request (journaling
    filesystems are simply waiting until all writes are finished before
    continueing, right?).

    There are same cases (in device-mapper) where

    a) writes get get suspended and queued for later submission where it is
    not ensured that those writes are submitted before any other writes that
    could possibly occur after the device gets resumed (generic dm code)
    b) a stack (instead of a fifo) is used to queue requests and submit them
    later (not yet included code)
    c) writes can get queued but reads are directly passed through
    (snapshotting code too)

    Also, if DM recevices a barrier shouldn't this barrier be somehow sent
    to all real devices instead of the one that the request is actually sent
    to?

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