[dm-devel] Re: device mapper not reporting no-barrier-support?

[Alasdair G Kergon]

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 at 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 at redhat.com