Re: data corruption using raid0+lvm2+jfs with 2.6.0-test3

From: Neil Brown <neilb@cse.unsw.edu.au>
To: Mike Fedyk <mfedyk@matchmail.com>
Cc: Joe Thornber <thornber@sistina.com>,
	Andrew Morton <akpm@osdl.org>,
	Tupshin Harper <tupshin@tupshin.com>,
	linux-kernel@vger.kernel.org, Jens Axboe <axboe@suse.de>
Subject: Re: data corruption using raid0+lvm2+jfs with 2.6.0-test3
Date: Sun, 17 Aug 2003 10:12:27 +1000	[thread overview]
Message-ID: <16190.51307.990399.306100@gargle.gargle.HOWL> (raw)
In-Reply-To: message from Mike Fedyk on Saturday August 16

On Saturday August 16, mfedyk@matchmail.com wrote:
> On Sat, Aug 16, 2003 at 06:00:21PM +1000, Neil Brown wrote:
> > On Friday August 15, mfedyk@matchmail.com wrote:
> > > On Wed, Aug 13, 2003 at 09:05:58AM +1000, Neil Brown wrote:
> > > > On Tuesday August 12, akpm@osdl.org wrote:
> > > > > Tupshin Harper <tupshin@tupshin.com> wrote:
> > > > > >
> > > > > > raid0_make_request bug: can't convert block across chunks or bigger than 
> > > > > > 8k 12436792 8
> > 
> > > > 
> > > > Probably the simplest solution to this is to put in calls to
> > > > bio_split, which will need to be strengthed to handle multi-page bios.
> > > > 
> > > > The policy would be:
> > > >   "a client of a block device *should* honour the various bio size 
> > > >    restrictions, and may suffer performance loss if it doesn't;
> > > >    a block device driver *must* handle any bio it is passed, and may
> > > >    call bio_split to help out".
> > > > 
> > > 
> > > Any progress on this?
> > 
> > No, and I doubt there will be in a big hurry, unless I come up with an
> > easy way to make lvm-over-raid0 break instantly instead of eventually.
> > 
> > I think that for now you should assume tat lvm over raid0 (or raid0
> > over lvm) simply isn't supported.  As lvm (aka dm) supports striping,
> > it shouldn't be needed.
> 
> I have a raid5 with "4" 18gb drives, and one of the "drives" is two 9gb
> drives in a linear md "array".
> 
> I'm guessing this will hit this bug too?

This should be safe.  raid5 only ever submits 1-page (4K) requests
that are page aligned, and linear arrays will have the boundary
between drives 4k aligned (actually "chunksize" aligned, and chunksize
is atleast 4k). 

So raid5 should be safe over everything (unless dm allows striping
with a chunk size less than pagesize).

Thinks: as an interim solution of other raid levels - if the
underlying device has a merge_bvec_function which is being ignored, we
could set max_sectors to PAGE_SIZE/512.  This should be safe, though
possibly not optimal (but "safe" is trumps "optimal" any day).

NeilBrown
> 
> I have a couple systems that use software raid5 that I'll avoid putting
> 2.6-test on until I know the raid is more reliable (or is this only with
> md+lvm?)