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

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Message-ID: <476188C4.9030802@rtr.ca>
Date:	Thu, 13 Dec 2007 14:32:20 -0500
From:	Mark Lord <lkml@....ca>
To:	Mark Lord <liml@....ca>
Cc:	Jens Axboe <jens.axboe@...cle.com>,
	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 ?

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! 

Cheers
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