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    Subjectdevice mapper not reporting no-barrier-support?

    I'm currently stuck between Kernel LVM and DRBD, as I'm using Kernel with DRBD 8.2.5 on top of an LVM2 device (LV).

    -LVM2/device mapper doesn't support write barriers
    -DRBD uses blkdev_issue_flush() to flush its metadata to disk.
    On a no-barrier-device, DRBD should receive EOPNOTSUPP, but
    it really does receive an EIO. Promptly, DRBD gives the
    error message "drbd0: local disk flush failed with status -5".

    The physical disk (in LVM speak) is a RAID1 on a 3ware 9650SE-2LP
    controller; the driver 3w-9xxx supports barriers and after moving my D
    RBD device from the LV to a single partition on the same RAID1, the
    error messages from DRBD vanished.

    I've posted a lengty summary of my findings to

    ... where Lars Ellenberg from DRBD basically responded in

    ... that DRBD does catch the EOPNOTSUPP for blkdev_issue_flush and
    BIO_RW_BARRIER, but the lvm implementation of blkdev_issue_flush in aparently does return EIO for blkdev_issue_flush.

    So simply the question: how should a top-layer driver check wether a lower
    device does support barriers? md-raid does check this way differently than
    e.g. XFS does, while DRBD also adds a third way to check this.
    Or is this "merely" a bug in drivers/md/dm.c?

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