Inconsistent IO performance

Fri Aug 13 23:29:56 UTC 2010

On Fri, Aug 13, 2010 at 09:01:09AM -0700, Kevin Oberman wrote:
> For some time I have seen very odd issues with IO performance on
> 8-Stable. Going back to November of last year when 8.0 was released, I
> see variations of up to 22% in identical operations. This is not a
> degradation as the performance moves up and down.
> 
> This is a very simplistic case. I have two identical disks (Fujitsu 80G)
> on a ThinkPad T43 with a 2 GHz CPU and 2G RAM. I run the command:
> dd bs=516096 if=/dev/ad0 of=/dev/ad2
> 
> I do this in single user mode immediately after a boot with no disks
> mounted for write. Just a 'boot -s', ,Enter> to start the shell, and the
> dd. I would expect very consistent performance from run to run, but I
> don't get it. Here are the results since 8.0 was released:
>  Date   Xfer rate       Kernel date
> 12/4/09	19,242,573	Nov. 26 kernel (8.0-stable)
> 12/9/09	18,304,565	Dec. 6 kernel
> 12/17/0923,676,086	
> 1/5/10	18,648,609	
> 1/14/10	23,488,540	Jan. 6 kernel
> 1/21/10	19,551,680	Jan. 15 kernel
> 1/27/10	21,176,385	Jan. 21 kernel
> 2/5/10	22,387,745	
> 2/11/10	23,387,894	
> 2/17/10	20,412,172	Feb. 16 kernel
> 2/25/10	22,049,128	
> 3/4/10	22,099,624	Mar. 3 kernel
> 3/17/10	20,334,896	Mar. 3 kernel
> 3/31/10	21,655,213	Mar. 25 kernel
> 4/8/10	19,673,170	
> 4/14/10	22,235,518	
> 4/30/10	21,262,223	Apr. 14 kernel
> 6/3/10	22,838,125	May 24 kernel
> 6/17/10	18,481,270	
> 6/28/10	20,958,356	
> 7/8/10	19,698,282	June 28 kernel
> 7/21/10	23,330,556	
> 7/28/10	20,544,392	July 24 kernel (8.1-stable)
> 8/13/10	22,093,259	Aug. 9 kernel
> 
> Note the dramatic differences even on the same kernel. For the December
> 6 kernel, for example, I see a maximum of 23,676,086 and a minimum of
> just 18,304,565. ????
> 
> Can anyone explain what might be causing such a dramatic difference?
> 
> I should also note that the system was consistent back in V6 and V7
> days. Consistently slow, but consistent. 17.5M was the norm in V6 and
> 18.0M in V7. The performance jumped to about 19M in March of 09 and jumped
> to its current speeds with 8.0. So performance has greatly improved to
> where the slowest times are better than the fastest prior to March of
> 09. Just very inconsistent.
> 
> I don't know that anything is wrong, but I'd love to understand why this
> is happening.

The system in question is a Thinkpad T43 laptop[1], which is from circa
2005 and uses an ICH6-M southbridge (note the -M).  We don't know the
exact model of Fujitsu hard disk used, but since it's a laptop my guess
is that it's 5400rpm, and PATA.

System drastically differs on laptops if being powered off the battery
vs. AC.  Were the tests performed consistently with the exact same setup
(which: battery or AC?) every time?  Given that the system role is a
laptop, I imagine not.

Can you also provide output from these commands?

- atacontrol list
- atacontrol info ataX  (where X is the channel number the ad0 drive
  is connected to)
- atacontrol cap ad0

Anyway, I would expect the system to be seeing 50-60MB/sec, but I'm
pulling those numbers out of thin air.  An ICH6-M may be "old", but keep
reading for a comparison system that's even older...

The deviation in your disk I/O isn't a major surprise (to me anyway),
given the system specs.  What *does* surprise me is your abysmal I/O
speeds in general.  18MB/sec min, 24MB/sec max?!  ICH6-M can do a lot
more than that.  Something isn't right.

It sounds to me like the disk itself has some kind of internal problem
(cache that's gone bad, something mechanical that isn't audible, etc.);
even for a 5400rpm drive those numbers are very low.  Other
possibilities include a southbridge that's going bad, or some kind of
power-related problem that's causing the drive to spin at a lower speed
than 5400rpm (though SMART sometimes can notice this).  I'm grasping at
straws with this one, but excessive dust can slow a system down (from
what I'm told by EE folks; something about more electricity being
required to push voltage across a trace...)

Now the comparison -- here's a system that's way older than yours.

- FreeBSD 6.4-STABLE, i386
- Supermicro SuperServer 5010E [2]
- Intel Pentium 3 (not sure of speed)
- 1GB RAM
- Intel ICH2 southbridge
- ad0: Maxtor STM3160815A disk (160GB, 8MB cache, 7200rpm, ATA133)
- ad1: Maxtor STM3160815A disk (160GB, 8MB cache, 7200rpm, ATA133)
- Disk connected via ICH2
- System in multi-user with some light load
- Command #1: dd if=/dev/ad0 of=/dev/null bs=64k count=100000
- Command #2: dd if=/dev/ad1 of=/dev/null bs=64k count=100000
- Commands run 4 times in succession

Result for command #1:

6553600000 bytes transferred in 84.110282 secs (77916752 bytes/sec)
6553600000 bytes transferred in 84.197506 secs (77836035 bytes/sec)
6553600000 bytes transferred in 84.205020 secs (77829089 bytes/sec)
6553600000 bytes transferred in 84.662426 secs (77408602 bytes/sec)

Result for command #2:

6553600000 bytes transferred in 85.052923 secs (77053201 bytes/sec)
6553600000 bytes transferred in 85.040286 secs (77064652 bytes/sec)
6553600000 bytes transferred in 85.040805 secs (77064181 bytes/sec)
6553600000 bytes transferred in 85.040560 secs (77064403 bytes/sec)

My recommendation: start looking at replacing hardware.

Start by replacing the hard disk with something newer and see what
becomes of that.  If you want recommendations: if power draw, noise, and
thermals are a concern point for you, get a 5400rpm drive.  Try to find
something with 16MB cache or more.  The WD Scorpio Blue drives are
supposedly quite nice, but I don't know if they come in PATA.

I'm not particularly fond of Fujitsu drives (my day job consists of
watching their SCSI disks freak out in random ways), so I'd choose to
stay away from those.  Same goes for Toshiba (just recently replaced one
of those on a laptop; cache went bad, disk I/O was absurd).

If you want something totally crazy to try, how about booting a FreeBSD
7.2 or 7.3 LiveFS CD and doing your dd's?

[1]: http://www.thinkwiki.org/wiki/Category:T43
[2]: http://www.supermicro.com/products/system/1U/5010/SYS-5010E.cfm

-- 
| Jeremy Chadwick                                   jdc at parodius.com |
| Parodius Networking                       http://www.parodius.com/ |
| UNIX Systems Administrator                  Mountain View, CA, USA |
| Making life hard for others since 1977.              PGP: 4BD6C0CB |