Watching performance on a DHCP Server
Frank Bulk
frnkblk at iname.com
Tue Feb 12 05:16:49 UTC 2008
We use a variety of lease times:
- 3 days for our PPPoA/PPPoE/BWA CPE/cable modem CPE
- 3 days for our cable modems
- 2 weeks for our eMTAs
- one month for our IP TV STBs
Frank
_____
From: dhcp-users-bounce at isc.org [mailto:dhcp-users-bounce at isc.org] On Behalf
Of Blake Hudson
Sent: Monday, February 11, 2008 9:51 AM
To: dhcp-users at isc.org
Subject: Re: Watching performance on a DHCP Server
Thanks Barr, it is always interesting to hear relative practical
experiences. This is exactly the kind of problem I would like to
prepare/plan for. I've read that Microsoft defaults to an 8 day lease time.
ISC uses a default lease time of 10 minutes, with a max of 2 hours in their
sample config included with 4.1.x.
We have successfully used 1 day leases in the past. Though I know some
larger ISPs use 5 day, 7 day or even longer lease times.
I'm assuming that the main advantage to a short lease time is that hosts
that join and leave a network give their leases up more rapidly (keeping IP
pool usage as low as possible). The main advantage to longer lease times
being load on the DHCP server. If I have a relatively stable network (only
known macs are allowed) then it seems like a longer lease time (say 7-14
days) is more appropriate. And on a relatively stable cable or DSL network
anything between 5-7 days seems acceptable? Volatile networks (wifi
hotspots?) would probably benefit from a 1 hour or shorter lease time.
Does it sound like I am in the right ballpark with these figures?
-Blake
-------- Original Message --------
Subject: Re: Watching performance on a DHCP Server
From: Barr Hibbs <mailto:rbhibbs at pacbell.net> <rbhibbs at pacbell.net>
To: dhcp-users at isc.org
Date: Sunday, February 10, 2008 4:35:37 PM
this experience is with a derivative of version 2 of the
server, but as the basic functionality has not changed
significantly for IPv4, it may be instructive....
at the time, our environment had about 12,000 clients split
roughly 55/45 between two servers... each server was
connected by two links to each of approximately 120 remote
subnets, each link diversely routed to minimize disruption
due to network problems, but also delivering 2 copies of
every client message to the servers
we suffered a massive regional power failure that lasted
2-1/2 days before complete restoration... our clients
received 7-day leases, largely grouped with their renewal
times between 8 am and 6 pm, so in a 2-1/2 day outage, we
could expect renewal requests to come from about half of our
clients, and certainly init-reboot requests to come from
all... so, that is roughly 18,000 requests to be serviced
as power is restored....
of course, the power restoral didn't occur all at once, but
was somewhat randomly distributed over a period of roughly
32 hours
entirely by coincidence, we had instrumented the server to
capture detailed message arrival rates and response times,
expecting a normal, boring weekend... but then the power
failed, and... we got lots more data than we expected!
the real-time clock on our computers was capable of only 1
millisecond resolution, so I must extrapolate.... our
servers survived a nearly CONTINUOUS load of more than 1,000
requests per second for 32 hours...
of course, your mileage may vary, but by choosing an
appropriate lease lifetime, you will probably see similar or
better performance.
--Barr Hibbs
-----Original Message-----
From: dhcp-users-bounce at isc.org
[mailto:dhcp-users-bounce at isc.org]On
Behalf Of David W. Hankins
Sent: Friday, February 08, 2008 08:55
To: dhcp-users at isc.org
Subject: Re: Watching performance on a DHCP Server
On Thu, Feb 07, 2008 at 06:07:51PM -0600, Blake
Hudson wrote:
By default in my distribution the leases file
is stored in
/var/lib/dhcpd/dhcpd.leases. This happens to be
on a RAID1 array with
15k scsi disks and iostat shows the array as
being maxed out once it
reaches ~ 300 I/O's per second. DHCP logging is
done asynchronously to
the same array (which normally experiences ~ 50
I/O ops). With CPU and
memory barely breaking a sweat, this leads me
to believe that the
limitation is with the disks (lots of tiny writes).
I could move the leases file to a different
array, or to tmpfs, but
before I do I just want to know if these
results are typical and that I
have interpreted the test data correctly and
made the correct
determination as to the bottleneck.
those results are typical for that kind of
hardware, and you have
interpreted the test data correctly: fsync() is
the biggest
bottleneck.
in 4.1.0a1, you will find a feature, however,
which was provided to
us in a patch by Christof Chen. it permits the
server to queue
multiple ACKs behind a single fsync(); default 28
(576 byte DHCP
packets filling default socket buffer send
sizes). the burst of acks
are sent presently if the sockets go dry, and
shortly will be backed
up with a sub-second timeout.
it has some bugs we're working on, particularly
with failover, but
we'll address those in alpha.
you may find that it provides some form of
multiplicative benefit to
your performance stats, since fsync() is the
bottleneck, and now there
are 28 acks per fsync max.
so if you are only pushing 50 requests/s
currently, you may live
comfortably in a 250 request/s buffer for some
months until the
4.1.x code is stable?
Also, I would appreciate any anecdotal evidence
with regards to how many
requests are typical in a large network under
normal (or abnormal)
conditions. If 10,000 users all of a sudden
came online, how many
requests would they really generate per second?
there have been a few folks who suffered mass
power outages, i don't
know what search query to use, but you can find
them on the old
dhcp-server mailing list. they did not report
problems, rather the
surprise at the lack of problem.
--
Ash bugud-gul durbatuluk agh burzum-ishi krimpatul.
Why settle for the lesser evil?
https://secure.isc.org/store/t-shirt/
--
David W. Hankins "If you don't do it right the first time,
Software Engineer you'll just have to do it again."
Internet Systems Consortium, Inc. -- Jack T. Hankins
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