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    SubjectRe: [dm-devel] Re: device mapper not reporting no-barrier-support?
    On Tue, Feb 26 2008, Anders Henke wrote:
    > On Tue, Feb 26 2008 schrieb Jens Axboe:
    > > On Tue, Feb 26 2008, Anders Henke wrote:
    > > > On Tue, Feb 26 2008 Jens Axboe wrote:
    > > > > On Tue, Feb 26 2008, Alasdair G Kergon wrote:
    > > > > > 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;
    > > > >
    > > > > You are right, the return value got broken there. Does this make it
    > > > > return -EOPNOTSUPP properly for you?
    > > >
    > > >
    > > > No, it doesn't.
    > > >
    > > >
    > > >
    > > > I've applied your patch manually, as 2.6.24.2. doesn't have a "blk-barrier.c":
    > > >
    > > > ---cut
    > > > --- linux-2.6.24.2/block/ll_rw_blk.c.prepatch 2008-02-11
    > > > 06:51:11.000000000 +0100
    > > > +++ linux-2.6.24.2/block/ll_rw_blk.c 2008-02-26 20:02:28.514641620
    > > > +0100
    > > > @@ -2667,8 +2667,11 @@
    > > >
    > > > static void bio_end_empty_barrier(struct bio *bio, int err)
    > > > {
    > > > - if (err)
    > > > + if (err) {
    > > > + if (err == -EOPNOTSUPP)
    > > > + set_bit(BIO_EOPNOTSUPP, &bio->bi_flags);
    > > > clear_bit(BIO_UPTODATE, &bio->bi_flags);
    > > > + }
    > > >
    > > > complete(bio->bi_private);
    > > > }
    > > > ---cut
    > > >
    > > > ... and the resulting kernel shows exactly the same behaviour than before:
    > >
    > > Not surprising, as you missed half of the patch:
    >
    > Ouch. Thank you for pointing this out.
    >
    > I've been spending too much time of the day with things who have a negative
    > impact on my concentration and I shouldn't manually patch kernels at
    > this time of the day.
    >
    > Yes, it's useless to set a bit, but never check it (like in my version of
    > your patch).
    >
    > After adding the second part of your patch, the resulting kernel works as
    > intended:
    >
    > [ 234.946192] drbd0: conn( WFSyncUUID -> SyncTarget )
    > [ 234.956176] drbd0: Began resync as SyncTarget (will sync 19542404 KB
    > [4885601
    > bits set]).
    > [ 234.972567] drbd0: Writing meta data super block now.
    > [ 235.018203] drbd0: local disk flush failed with status -95
    >
    > DRBD sees the EOPNOTSUPP, logs this message only once and doesn't try
    > any further barrier requests (as intended).

    OK good, that's what I expected :-)

    I'll queue the patch for 2.6.25, the 2.6.24 should go to stable. Send me
    a properly formatted patch and I'll make sure it goes that way.

    Thanks for testing!

    --
    Jens Axboe



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