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Date: Sun, 28 Mar 2004 10:15:26 +1000
From: Nick Piggin <nickpiggin@yahoo.com.au>
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To: Jeff Garzik <jgarzik@pobox.com>
CC: linux-ide@vger.kernel.org, Linux Kernel <linux-kernel@vger.kernel.org>,
       Andrew Morton <akpm@osdl.org>
Subject: Re: [PATCH] speed up SATA
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Jeff Garzik wrote:
> Jeff Garzik wrote:
> 
>> It's up to the sysadmin to choose a disk scheduling policy they like, 
>> which implies that a _scheduler_, not each individual driver, should 
>> place policy limitations on max_sectors.
> 
> 
> 
> <tangent>
> 
> The block layer / scheduler guys should also think about a general (not 
> SCSI specific) way to tune TCQ tag depth.  That's IMO another policy 
> decision.
> 
> I'm about to add a raft of SATA-2 hardware, all of which are queued. The 
> standard depth is 32, but one board supports a whopping depth of 256.
> 
> Given past discussion on the topic, you probably don't want to queue 256 
> requests at a time to hardware :)  But the sysadmin should be allowed 
> to, if they wish...

I think you could limit the number of in-flight requests quite
easily, even for drivers that do not use the block layer's
queueing functions.

Aside, you should make 2 or 4 tags the default though: that
still gives you the pipelining without sacrificing latency
and usibility.

The only area (I think) where large queues outperform the IO
scheduler are lots of parallel, scattered reads. I think this
is because the drive can immediately return cached data even
though it looks like a large seek to the IO scheduler.
(This is from testing on my single, old SCSI disk though.)
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