Watching performance on a DHCP Server
blake at ispn.net
Mon Feb 11 22:37:57 UTC 2008
-------- Original Message --------
Subject: Re: Watching performance on a DHCP Server
From: Olaf van der Spek <olafvdspek at gmail.com>
To: dhcp-users at isc.org
Date: Monday, February 11, 2008 3:55:24 PM
> On Feb 11, 2008 7:49 PM, Blake Hudson <blake at ispn.net> wrote:
>> You may want to review the thread from the beginning. My network currently
>> has 10,000+ DHCP clients (and I plan on accommodating double that within the
>> lifetime of this server). I have a beefy server (4x 3.0GHz Xeon, 2x 15k
>> RAID1) and it was only able to reliably handle 10 to 20 4-way discover
> Isn't that too much CPU power, too little HDD power for this job?
The server has eight directly attached 147GB 15k drives... it just
happens that the leases file is on a RAID 1 array. Without fsyncs, the
next limiting factor is CPU power... which I hit very quickly at around
40 4-way handshakes a second (unfortunately dhcpd is not multithreaded).
I have not been able to accurately measure 2-way handshakes after
removing the fsync code. I'm guessing it's 5,000 or more per second...
way more than I need.
>> handshakes a second, 2-way handshakes were maybe double or triple those
>> numbers. When pounded by DHCP requests, it's possible that even less are
>> processed in a timely manner due to collisions, timeouts, etc.
>> The potential convergence time concerns me if there were an enterprise wide
>> prolonged outage. Increasing the lease time means less leases processed on a
>> regular basis (not as much of a concern) as well as reducing the need for a
>> 4 way handshake and being able to use a 2 way handshake if a prolonged
>> outage occurs (my main concern I'd like to accommodate).
> I just read the thread and the disk IO indeed changes the game. ;)
> Still, the gap between theoretical top performance and actual
> performance seems extreme.
> Even if you have the requirement that leases have to be committed to
> disk before you respond to the client.
fsyncs are the most I/O intensive operation you can perform on magnetic
media. If I remember correctly it takes at least 2 full rotations of the
disk for an fsync to occur, although it may take longer to commit more
data. This is were solid state rules over magnetic storage.
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