Date:	Mon, 30 Mar 2009 14:39:05 -0400
From:	Mark Lord <lkml@....ca>
To:	Chris Mason <chris.mason@...cle.com>
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

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.

> It took a little experimentation, and I had to switch to the noop
> scheduler (no idea why).  
> 
> Also, I had to watch vmstat closely.  When the test first started,
> vmstat was reporting 500kb/s or so write throughput.  After the test ran
> for a few minutes, vmstat jumped up to 8MB/s.
> 
> My guess is that the drive has some internal threshold for when it
> decides to only write in cache.  The switch to 8MB/s is when it switched
> to cache only goodness.  Or perhaps the attached program is buggy and
> I'll end up looking silly...it was some quick coding.
> 
> The test forks two procs.  One proc does 4k writes to the first 26MB of
> the test file (/dev/sdb for me).  These writes are O_DIRECT, and use a
> block size of 4k.
> 
> The idea is that we fill the cache with work that is very beneficial to
> keep in cache, but that the drive will tend to flush out because it is
> filling up tracks.
> 
> The second proc O_DIRECT writes to two adjacent sectors far away from
> the hot writes from the first proc, and it puts in a timestamp from just
> before the write.  Every second or so, this timestamp is printed to
> stderr.  The drive will want to keep these two sectors in cache because
> we are constantly overwriting them.
> 
> (It's worth mentioning this is a destructive test.  Running it
> on /dev/sdb will overwrite the first 64MB of the drive!!!!)
> 
> Sample output:
> 
> # ./wb-latency /dev/sdb
> Found tv 1238434622.461527
> starting hot writes run
> starting tester run
> current time 1238435045.529751
> current time 1238435046.531250
> ...
> current time 1238435063.772456
> current time 1238435064.788639
> current time 1238435065.814101
> current time 1238435066.847704
> 
> Right here, I pull the power cord.  The box comes back up, and I run:
> 
> # ./wb-latency -c /dev/sdb
> Found tv 1238435067.347829
> 
> When -c is passed, it just reads the timestamp out of the timestamp
> block and exits.  You compare this value with the value printed just
> before you pulled the block.
> 
> For the run here, the two values are within .5s of each other.  The
> tester only prints the time every one second, so anything that close is
> very good.  I had pulled the plug before the drive got into that fast
> 8MB/s mode, so the drive was doing a pretty good job of fairly servicing
> the cache.
> 
> My drive has a cache of 32MB.  Smaller caches probably need a smaller
> hot zone.
> 
> -chris
> 
> 

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