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    SubjectRe: [dm-devel] Re: device mapper not reporting no-barrier-support?
    On Mon, Feb 25, 2008 at 03:20:50PM -0800, Andrew Morton wrote:
    > On Mon, 25 Feb 2008 14:26:15 +0100 Anders Henke <anders.henke@1und1.de> wrote:
    > > I'm currently stuck between Kernel LVM and DRBD, as I'm using Kernel
    > > 2.6.24.2 with DRBD 8.2.5 on top of an LVM2 device (LV).
    > > -LVM2/device mapper doesn't support write barriers

    That's right.

    > > -DRBD uses blkdev_issue_flush() to flush its metadata to disk.

    Which won't work if device-mapper is underneath.

    > > 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".
    > > I've posted a lengty summary of my findings to
    > > http://lists.linbit.com/pipermail/drbd-user/2008-February/008665.html
    > > ... that DRBD does catch the EOPNOTSUPP for blkdev_issue_flush and
    > > BIO_RW_BARRIER, but the lvm implementation of blkdev_issue_flush in
    > > 2.6.24.2 aparently does return EIO for blkdev_issue_flush.
    > I'd say it's a DM bug.

    The dm code is unchanged, but look at the limited endio handling in
    ll_rw_blk.c:

    static void bio_end_empty_barrier(struct bio *bio, int err)
    {
    if (err)
    clear_bit(BIO_UPTODATE, &bio->bi_flags);

    complete(bio->bi_private);
    }

    int blkdev_issue_flush(struct block_device *bdev, sector_t *error_sector)
    {
    ...
    wait_for_completion(&wait);
    if (error_sector)
    *error_sector = bio->bi_sector;
    ret = 0;
    if (!bio_flagged(bio, BIO_UPTODATE))
    ret = -EIO;

    Alasdair
    --
    agk@redhat.com


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