linux-kernel - Barriers still not passing on simple dm devices...

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Date:	Mon, 23 Mar 2009 14:04:28 -0500
From:	Eric Sandeen <sandeen@...hat.com>
To:	device-mapper development <dm-devel@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
CC:	Jens Axboe <jens.axboe@...cle.com>, Andi Kleen <ak@...e.de>,
	"MASON,CHRISTOPHER" <CHRIS.MASON@...cle.com>
Subject: Barriers still not passing on simple dm devices...

I've noticed that on 2.6.29-rcX, with Andi's patch
(ab4c1424882be9cd70b89abf2b484add355712fa, dm: support barriers on
simple devices) barriers are still getting rejected on these simple devices.

The problem is in __generic_make_request():

                if (bio_barrier(bio) && bio_has_data(bio) &&
                    (q->next_ordered == QUEUE_ORDERED_NONE)) {
                        err = -EOPNOTSUPP;
                        goto end_io;
                }

and dm isn't flagging its queue as supporting ordered writes, so it's
rejected here.

Doing something like this:

+ if (t->barriers_supported)
+         blk_queue_ordered(q, QUEUE_ORDERED_DRAIN, NULL);

somewhere in dm (I stuck it in dm_table_set_restrictions() - almost
certainly the wrong thing to do) did get my dm-linear device to mount
with xfs, w/o xfs complaining that its mount-time barrier tests failed.

So what's the right way around this?  What should dm (or md for that
matter) advertise on their queues about ordered-ness?  Should there be
some sort of "QUEUE_ORDERED_PASSTHROUGH" or something to say "this level
doesn't care, ask the next level" or somesuch?  Or should it inherit the
flag from the next level down?  Ideas?

Thanks,
-Eric
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