slow directory listing

[Ron Peterson <rpeterso@mtholyoke.edu>]

I'm setting up a new mail server, and am testing/tweaking IO.  I have
two directories: /test/a which contains 750 mbox files totalling 8GB,
and /test/a2, which contains the exact same number of files, same names,
all zero length.

I am using ext3.  I have done this experiment with both indexed and
non-indexed directories (mke2fs -O dir_index ...).  I have also tried
setting the noatime mount option.

The times taken to do a directory listing are significantly different.

1037# time ls /test/a2 > /dev/null

real    0m0.006s
user    0m0.000s
sys     0m0.006s

1038# time ls /test/a > /dev/null

real    0m5.244s
user    0m4.875s
sys     0m0.346s

If I refer to a specific file, there's still a difference, but only 5x,
vs. 875x above.

1044# time ls a/anmbox > /dev/null

real    0m0.010s
user    0m0.009s
sys     0m0.002s

Fri Jun 10 10:31:02 root@slush:/db/tmp
1045# time ls a2/anmbox > /dev/null

real    0m0.002s
user    0m0.001s
sys     0m0.001s

I'm assuming this is normal behaviour.  (?)  However, I'd like to
understand what's happening a little better, and I'm wondering if
there's anything I'm overlooking vis-a-vis tuning my filesystem properly
for this type of application.

Linux 2.6.11.11 on Debian Sarge.  Dell 2800 w/ LSI Megaraid on PCI/E to
Utra320 SCSI disks.

-- 
Ron Peterson
Network & Systems Manager
Mount Holyoke College
http://www.mtholyoke.edu/~rpeterso