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    SubjectRe: [dm-devel] Re: [RFD] BIO_RW_BARRIER - what it means for devices, filesystems, and dm/md.
    On Mon, May 28, 2007 at 11:30:32AM +1000, Neil Brown wrote:
    > 1/ A BIO_RW_BARRIER request should never fail with -EOPNOTSUP.

    The device-mapper position has always been that we require

    > a zero-length BIO_RW_BARRIER

    (i.e. containing no data to read or write - or emulated, possibly
    device-specific)

    before we can provide full barrier support.
    (Consider multiple active paths - each must see barrier.)

    Until every device supports barriers -EOPNOTSUP support is required.
    (Consider reconfiguration of stacks of devices - barrier support is a
    dynamic block device property that can switch between available and
    unavailable at any time.)

    Alasdair
    --
    agk@redhat.com
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