linux-kernel - Re: Linux 2.6.29

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Date:	Mon, 30 Mar 2009 14:52:29 -0400
From:	Chris Mason <chris.mason@...cle.com>
To:	Mark Lord <lkml@....ca>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	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, 2009-03-30 at 14:39 -0400, Mark Lord wrote:
> Chris Mason wrote:
> >
> > 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.
> ..
> 
> I'd be more interested in how you managed that (above),
> than the quite different test you describe below.
> 
> Yes, different, I think.  The test below just times how long a single
> chunk of data might stay in-drive cache under constant load,
> rather than how long it takes to flush the drive cache on command.
> 
> Right?
> 
> Still, useful for other stuff.
> 

That's right, it is testing for starvation in a single sector, not for
how long the cache flush actually takes.  But, your remark from higher
up in the thread was this:

        > 
        > Anything in the drive's write cache very probably made 
        > it to the media within a second or two of arriving there.
        >
        
Sorry if I misread things.  But the goal is just to show that it really
does matter if we use a writeback cache with or without barriers.  The
test has two datasets:

1) An area that is constantly overwritten sequentially
2) A single sector that stores a critical bit of data.

#1 is the filesystem log, #2 is the filesystem super.  This isn't a
specialized workload ;)

-chris


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