linux-kernel - Re: Lower HD transfer rate with NCQ enabled?

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Date:	Mon, 09 Apr 2007 23:58:55 -0400
From:	Mark Lord <lkml@....ca>
To:	Phillip Susi <psusi@....rr.com>
Cc:	Chris Snook <csnook@...hat.com>, Paa Paa <paapaa125@...mail.com>,
	linux-kernel@...r.kernel.org
Subject: Re: Lower HD transfer rate with NCQ enabled?

Phillip Susi wrote:
> Mark Lord wrote:
>> Phillip Susi wrote:
>>> Sounds like this is a serious bug in the WD firmware.
>>
>> For personal systems, yes.  For servers, probably not a bug.
>>
>> Disabling readahead means faster execution queued commands,
>> since it doesn't have to "linger" and do unwanted read-ahead.
>> So this bug is a "feature" for random access servers.
>> And a big nuisance for everything else.
> 
> I think you misunderstand the bug.  The bug is not that the drive 
> disables internal readahead; the bug is that host supplied readahead 
> requests work so horribly.  It is a good thing that the drive allows the 
> host to control the readahead, but something is wrong if the drive's 
> readahead is WAY better than any the host can perform.

Well, in this case, it has already been determined that switching
to a different Linux I/O scheduler gives back most of the performance.

But the drive can do readahead better than the OS:  With the OS,
everything is broken up into discrete requests, whereas with the 
drive firmware, it can continuously update it's readahead projections,
even in the midst of a command.  So it does have an advantage.

But again, only the WD Raptor seems to have serious problems here.
Other drives cope well with readahead + NCQ just fine.

Cheers
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