linux-kernel - Re: Linux 2.6.29

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Message-ID: <20090330185449.GD24960@edu.joroinen.fi>
Date:	Mon, 30 Mar 2009 21:54:49 +0300
From:	Pasi Kärkkäinen <pasik@....fi>
To:	Chris Mason <chris.mason@...cle.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Mark Lord <lkml@....ca>, Ric Wheeler <rwheeler@...hat.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

On Mon, Mar 30, 2009 at 01:57:12PM -0400, Chris Mason wrote:
> On Mon, 2009-03-30 at 09:58 -0700, 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).
> 
> I had some fun trying things with this, and I've been able to reliably
> trigger stalls in write cache of ~60 seconds on my seagate 500GB sata
> drive.  The worst I saw was 214 seconds.
> 
> It took a little experimentation, and I had to switch to the noop
> scheduler (no idea why).  
> 

I remember cfq having a bug (or a feature?) that prevents queue depths
deeper than 1.. so with noop you get more ios to the queue. 

-- Pasi
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