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    SubjectRe: [RFD] BIO_RW_BARRIER - what it means for devices, filesystems, and dm/md.
    On Fri, Jun 01 2007, Neil Brown wrote:
    > On Friday June 1, wrote:
    > > On Thu, May 31, 2007 at 02:31:21PM -0400, Phillip Susi wrote:
    > > > David Chinner wrote:
    > > > >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....
    > > >
    > > > So what if you want a synchronous write, but DON'T care about the order?
    > >
    > > submit_bio(WRITE_SYNC, bio);
    > >
    > > Already there, already used by XFS, JFS and direct I/O.
    > Are you sure?
    > You seem to be saying that WRITE_SYNC causes the write to be safe on
    > media before the request returns. That isn't my understanding.
    > I think (from comments near the definition and a quick grep through
    > the code) that WRITE_SYNC expedites the delivery of the request
    > through the elevator, but doesn't do anything special about getting it
    > onto the media.
    > It essentially say "Submit this request now, don't wait for more
    > request to bundle with it for better bandwidth utilisation"

    That is exactly right. WRITE_SYNC doesn't give any integrity guarentees,
    it's just makes sure it goes straight through the io scheduler.

    Jens Axboe

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