linux-kernel - Re: Linux 2.6.29

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Date:	Mon, 30 Mar 2009 13:29:57 -0400
From:	Mark Lord <lkml@....ca>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Ric Wheeler <rwheeler@...hat.com>,
	Chris Mason <chris.mason@...cle.com>,
	"Andreas T.Auer" <andreas.t.auer_lkml_73537@...us.ath.cx>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Theodore Tso <tytso@....edu>,
	Stefan Richter <stefanr@...6.in-berlin.de>,
	Jeff Garzik <jeff@...zik.org>,
	Matthew Garrett <mjg59@...f.ucam.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	David Rees <drees76@...il.com>, Jesper Krogh <jesper@...gh.cc>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Linux 2.6.29

Linus Torvalds wrote:
> 
> On Mon, 30 Mar 2009, Mark Lord wrote:
>> I spent an entire day recently, trying to see if I could significantly fill
>> up the 32MB cache on a 750GB Hitach SATA drive here.
>>
>> With deliberate/random write patterns, big and small, near and far,
>> I could not fill the drive with anything approaching a full second
>> of latent write-cache flush time.
>>
>> Not even close.  Which is a pity, because I really wanted to do some testing
>> related to a deep write cache.  But it just wouldn't happen.
>>
>> I tried this again on a 16MB cache of a Seagate drive, no difference.
>>
>> Bummer.  :)
> 
> Try it with laptop drives. You might get to a second, or at least hundreds 
> of ms (not counting the spinup delay if it went to sleep, obviously). You 
> probably tested desktop drives (that 750GB Hitachi one is not a low end 
> one, and I assume the Seagate one isn't either).
> 
> You'll have a much easier time getting long latencies when seeks take tens 
> of ms, and the platter rotates at some pitiful 3600rpm (ok, I guess those 
> drives are hard to find these days - I guess 4200rpm is the norm even for 
> 1.8" laptop harddrives).
> 
> And also - this is probably obvious to you, but it might not be 
> immediately obvious to everybody - make sure that you do have TCQ going, 
> and at full depth. If the drive supports TCQ (and they all do, these days) 
> it is quite possible that the drive firmware basically limits the write 
> caching to one segment per TCQ entry (or at least to something smallish).
..

Oh yes, absolute -- I tried with and without NCQ (the SATA replacement
for old-style TCQ), and with varying NCQ queue depths.  No luck keeping
the darned thing busy flushing afterwards for anything more than
perhaps a few hundred millseconds.  I wasn't really interested in anything
under a second, so I didn't measure it exactly though.

The older and/or slower notebook drives (4200rpm) tend to have smaller
onboard caches, too.  Which makes them difficult to fill.

I suspect I'd have much better "luck" with a slow-ish SSD that has
a largish write cache.  Dunno if those exist, and they'll have to get
cheaper before I pick one up to deliberately bash on.  :)

Cheers
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