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Date: Mon, 9 Mar 2009 17:04:16 +0100 (CET)
From: Wojciech Puchar <wojtek@wojtek.tensor.gdynia.pl>
To: Nicolas Haller <nicolas@boiteameuh.org>
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Cc: freebsd-questions@freebsd.org
Subject: Re: Busy disk and page fault
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> The server is overload, the disk is 100% busy with 250 write operations
> per second and a throuput of 6MB/s.
> My first idea is because of mass random access/write on the disk. But I also see
> the server can make 20k page fault per second.

what page fault? most page faults in FreeBSD doesn't mean disk access, 
just no mapping present in page tables, which gets mapped after the fault.

Only if page is actually not present in memory it is fetched from disk.

top shows in what state is a process.
if it's biord or biorw - it's doing disk/file I/O, not swapping.

that's about FreeBSD part - about postgress part ask on postgress mailing 
list. i don't use it so i can't help you.