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

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Message-ID: <4761883A.7050908@rtr.ca>
Date:	Thu, 13 Dec 2007 14:30:02 -0500
From:	Mark Lord <liml@....ca>
To:	Jens Axboe <jens.axboe@...cle.com>
Cc:	Mark Lord <lkml@....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 ?

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.

Cheers

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