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    SubjectRe: [RFD] BIO_RW_BARRIER - what it means for devices, filesystems, and dm/md.
    On Thu, May 31, 2007 at 08:26:45AM +0200, Jens Axboe wrote:
    > On Thu, May 31 2007, David Chinner wrote:
    > > IOWs, there are two parts to the problem:
    > >
    > > 1 - guaranteeing I/O ordering
    > > 2 - guaranteeing blocks are on persistent storage.
    > >
    > > Right now, a single barrier I/O is used to provide both of these
    > > guarantees. In most cases, all we really need to provide is 1); the
    > > need for 2) is a much rarer condition but still needs to be
    > > provided.
    > >
    > > > if I am understanding it correctly, the big win for barriers is that you
    > > > do NOT have to stop and wait until the data is on persistant media before
    > > > you can continue.
    > >
    > > Yes, if we define a barrier to only guarantee 1), then yes this
    > > would be a big win (esp. for XFS). But that requires all filesystems
    > > to handle sync writes differently, and sync_blockdev() needs to
    > > call blkdev_issue_flush() as well....
    > >
    > > So, what do we do here? Do we define a barrier I/O to only provide
    > > ordering, or do we define it to also provide persistent storage
    > > writeback? Whatever we decide, it needs to be documented....
    >
    > The block layer already has a notion of the two types of barriers, with
    > a very small amount of tweaking we could expose that. There's absolutely
    > zero reason we can't easily support both types of barriers.

    That sounds like a good idea - we can leave the existing
    WRITE_BARRIER behaviour unchanged and introduce a new WRITE_ORDERED
    behaviour that only guarantees ordering. The filesystem can then
    choose which to use where appropriate....

    Cheers,

    Dave.
    --
    Dave Chinner
    Principal Engineer
    SGI Australian Software Group
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