A specific example of a disk i/o problem

[Richard Mahlerwein]

>From: Dieter <freebsd at sopwith.solgatos.com>
>Subject: Re: A specific example of a disk i/o problem
>To: freebsd-performance at freebsd.org
>Date: Friday, October 2, 2009, 1:07 AM
>
>Updated demo, just to make sure:
>
># big_file is larger than main memory, on same disk as man (/usr)
>time man de        # get baseline time for man command without >competing i/o
>cat big_file > /dev/null   # flush man command and data from memory
>cat big_file > /dev/null & # generate i/o, attempt to use up bottleneck
>time man de      # see how much longer man takes with competing >i/o

Trying it on something that surely will have i/o issues: PIII 500 w/ 60 GB PATA drive and 128 MB RAM.  Blank minimal install + man pages right now.  I'm trying with a "file /etc/" command instead of trying to reliably time man... It seems the file command always hits the disk.  This should also make any sort of "flushing" unecessary, shouldn't it?

liebnitz# uname -a
FreeBSD xyz 7.2-RELEASE FreeBSD 7.2-RELEASE #0: Fri May  1 08:49:13 UTC 2009     root at walker.cse.buffalo.edu:/usr/obj/usr/src/sys/GENERIC  i386

liebnitz# time dd if=/dev/zero of=/usr/bigfile count=4096 bs=1M
4096+0 records in
4096+0 records out
4294967296 bytes transferred in 363.700967 secs (11809062 bytes/sec)
0.055u 83.971s 6:03.78 23.0%    25+1071k 128+34391io 0pf+0w

liebnitz# time file /etc/ [run 5 times]
4.052u 0.047s 0:04.14 98.7%     15+1085k 0+0io 0pf+0w
4.068u 0.031s 0:04.13 99.0%     16+1095k 0+0io 0pf+0w
4.052u 0.046s 0:04.13 99.0%     16+1101k 0+0io 0pf+0w
4.044u 0.054s 0:04.13 99.0%     16+1092k 0+0io 0pf+0w
4.052u 0.046s 0:04.13 99.0%     15+1091k 0+0io 0pf+0w

Ok, so that looks pretty repeatable.  Now...
liebnitz# cat /usr/bigfile > /dev/null &
[1] 37923
liebnitz# time file /etc/* [again run 5 times]
4.146u 0.038s 0:06.80 61.3%     15+1103k 87+0io 8pf+0w
4.128u 0.054s 0:06.73 61.9%     15+1095k 93+0io 6pf+0w
 




Flush as example.
liebnitz# cat /usr/bigfile > /dev/null
liebnitz# cat /usr/bigfile > /dev/null &
[1] 37890