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

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Date:	Thu, 13 Dec 2007 20:26:35 +0100
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Mark Lord <lkml@....ca>
Cc:	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:
> 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?

-- 
Jens Axboe

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