From owner-freebsd-performance@FreeBSD.ORG  Fri Sep 23 01:52:24 2005
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Date: Thu, 22 Sep 2005 21:52:23 -0400 (EDT)
From: Francisco Reyes <lists@natserv.com>
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Subject: Finding what's causing I/O
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Looking at vmstat I see the "b" colun never hits zero and it's usually 
between 5 and 20.

Is there a way to find out which program(s) are causing the I/O?

In some of the machines it was near trivial to find the culprit, but have 
a handfull of machines that I am not sure what the cause of I/O is.

The machines are mailservers so they likely will not hold any particular 
file open long.