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    SubjectRe: [dm-devel] Re: [RFD] BIO_RW_BARRIER - what it means for devices, filesystems, and dm/md.

    (dunny why you explicitly dropped me off the cc/to list when replying to
    my email, hence I missed it for 3 days)

    On Fri, May 25 2007, Phillip Susi wrote:
    > Jens Axboe wrote:
    > >A barrier write will include a flush, but it may also use the FUA bit to
    > >ensure data is on platter. So the only situation where a fallback from a
    > >barrier to flush would be valid, is if the device lied and told you it
    > >could do FUA but it could not and that is the reason why the barrier
    > >write failed. If that is the case, the block layer should stop using FUA
    > >and fallback to flush-write-flush. And if it does that, then there's
    > >never a valid reason to switch from using barrier writes to
    > >blkdev_issue_flush() since both methods would either both work or both
    > >fail.
    > IIRC, the FUA bit only forces THAT request to hit the platter before it
    > is completed; it does not flush any previous requests still sitting in
    > the write back queue. Because all io before the barrier must be on the
    > platter as well, setting the FUA bit on the barrier request means you
    > don't have to follow it with a flush, but you still have to precede it
    > with a flush.

    I'm well aware of how FUA works, hence the barrier FUA implementation
    does flush and then write-fua. The win compared to flush-write-flush is
    just a saved command, essentially.

    > >It's not block layer breakage, it's a device issue.
    > How isn't it block layer breakage? If the device does not support
    > barriers, isn't it the job of the block layer ( probably the scheduler )
    > to fall back to flush-write-flush?

    The problem is flush not working, the block layer can't fix that for you
    obviously. If it's FUA not working, the block layer should fall back to
    flush-write-flush, as they are obviously functionally equivalent.

    Jens Axboe

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      Last update: 2007-05-28 20:37    [W:0.022 / U:4.552 seconds]
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