linux-kernel - Re: CCISS performance drop in buffered disk reads in newer kernels

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Date:	Mon, 7 Dec 2009 20:32:54 +0100
From:	Jens Axboe <jens.axboe@...cle.com>
To:	"Miller, Mike (OS Dev)" <Mike.Miller@...com>
Cc:	Ozan Ça??layan <ozan@...dus.org.tr>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	"scameron@...rdog.cce.hp.com" <scameron@...rdog.cce.hp.com>
Subject: Re: CCISS performance drop in buffered disk reads in newer kernels

On Mon, Dec 07 2009, Miller, Mike (OS Dev) wrote:
> > > (max_hw_sectors_kb is 512 on my 2.6.25.20 setup and 1024 on 
> > 2.6.30.9 
> > > but it seems that it's read-only)
> > 
> > The *_hw_* values are the driver exported hardware limits, so 
> > they are always read-only.
> 
> Ahhh, I didn't know that. There is also an nr_requests attribute which
> to me implies limiting requests somewhere. The value of nr_request is
> 128 but the max commands to the cciss controllers exceed that value.
> What is nr_request supposed to do?

It controls what the block layer queue depth may be. As a rule of thumb,
it should be twice the hardware queue depth. A value of 128 means you
can have at most 128 reads and 128 writes queued in the IO scheduler. In
practice it's a bit more due to request allocation batching.

-- 
Jens Axboe

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