Watching performance on a DHCP Server
blake at ispn.net
Tue Feb 12 22:16:03 UTC 2008
-------- Original Message --------
Subject: Re: Watching performance on a DHCP Server
From: sthaug at nethelp.no
To: dhcp-users at isc.org
Date: Tuesday, February 12, 2008 3:20:50 PM
>> Th numbers I'm quoting are performance figures from the dhcperf testing
>> tool. If your 100k users were to be down 24 hours (due to regional power
>> failure, massive server/network failure), how would your DHCP server
>> cope with 100K requests at once? This thread is not about 'normal'
>> conditions, it is about the extreme cases that administrators (at least
>> good ones) are expected to design for and accommodate ahead of time.
> I'm planning for the extreme cases by having a redundant server (RAID,
> redundant power etc), and soon DHCP failover. I'm *not* planning for
> our DHCP infrastructure to cope with 100K requests at once, since I
> find such a scenario highly unlikely (even considering the failures
> that you have mentioned above).
> Steinar Haug, Nethelp consulting, sthaug at nethelp.no
I thought they were unlikely too, but I am choosing to plan for it since
I've seen two ISP's in the last month who have let 100% of their lease
expire. Their user bases are not 100k, but their servers/infrastructure
couldn't cope with the requests and it resulted in hours or days of
unnecessary downtime for their users. In these instances the problems
were likely 'administrator error', with either poor planning or
I'm glad that we have never experienced an issue of this magnitude, and
while administrator error can be mostly be reduced through redundancy,
planning, etc, there are some things that lie outside of our control. If
something should happen, I'd like to be prepared.
I'm preparing by testing our equipment and configuration in order to
confidently state what the server's limits are and to be able to provide
information which supports my claim. I felt the performance of DHCPD 3.x
was too slow. By removing the primary bottleneck in DHCPD (high numbers
of fsyncs) I have been able to increase the server's capacity ~3x on 4
way transactions, and ~100x on 2 way transactions while putting less
load on a server that provides other needed services. I feel confident
that if there were a large scale outage, our DHCP server would not be
overwhelmed, DHCP convergence would not be a limiting factor, and
customer downtime will have been minimized by these efforts.
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