linux-kernel - Re: QUEUE_FLAG_CLUSTER: not working in 2.6.24 ?

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Message-ID: <20071213193937.GG10104@kernel.dk>
Date:	Thu, 13 Dec 2007 20:39:37 +0100
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Mark Lord <lkml@....ca>
Cc:	Mark Lord <liml@....ca>, Matthew Wilcox <matthew@....cx>,
	IDE/ATA development list <linux-ide@...r.kernel.org>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	linux-scsi <linux-scsi@...r.kernel.org>
Subject: Re: QUEUE_FLAG_CLUSTER: not working in 2.6.24 ?

On Thu, Dec 13 2007, Mark Lord wrote:
> Mark Lord wrote:
> >Jens Axboe wrote:
> >>On Thu, Dec 13 2007, Mark Lord wrote:
> >>>Matthew Wilcox wrote:
> >>>>On Thu, Dec 13, 2007 at 01:48:18PM -0500, Mark Lord wrote:
> >>>>>Problem confirmed.  2.6.23.8 regularly generates segments up to 
> >>>>>64KB for libata,
> >>>>>but 2.6.24 uses only 4KB segments and a *few* 8KB segments.
> >>>>Just a suspicion ... could this be slab vs slub?  ie check your configs
> >>>>are the same / similar between the two kernels.
> >>>..
> >>>
> >>>Mmmm.. a good thought, that one.
> >>>But I just rechecked, and both have CONFIG_SLAB=y
> >>>
> >>>My guess is that something got changed around when Jens
> >>>reworked the block layer for 2.6.24.
> >>>I'm going to dig around in there now.
> >>
> >>I didn't rework the block layer for 2.6.24 :-). The core block layer
> >>changes since 2.6.23 are:
> >>
> >>- Support for empty barriers. Not a likely candidate.
> >>- Shared tag queue fixes. Totally unlikely.
> >>- sg chaining support. Not likely.
> >>- The bio changes from Neil. Of the bunch, the most likely suspects in
> >>  this area, since it changes some of the code involved with merges and
> >>  blk_rq_map_sg().
> >>- Lots of simple stuff, again very unlikely.
> >>
> >>Anyway, it sounds odd for this to be a block layer problem if you do see
> >>occasional segments being merged. So it sounds more like the input data
> >>having changed.
> >>
> >>Why not just bisect it?
> >..
> >
> >Because the early 2.6.24 series failed to boot on this machine
> >due to bugs in the block layer -- so the code that caused this regression
> >is probably in the stuff from before the kernels became usable here.
> ..
> 
> That sounds more harsh than intended --> the earlier 2.6.24 kernels (up to
> the first couple of -rc* ones failed here because of incompatibilities
> between the block/bio changes and libata.
> 
> That's better, I think! 

No worries, I didn't pick it up as harsh just as an odd conclusion :-)

If I were you, I'd just start from the first -rc that booted for you. If
THAT has the bug, then we'll think of something else. If you don't get
anywhere, I can run some tests tomorrow and see if I can reproduce it
here.

-- 
Jens Axboe

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