Server Sizing Question

[David W. Hankins]

On Tue, Oct 10, 2006 at 11:00:41AM -0500, David A. Evans wrote:
> a lot of differing opinions on it.    What I have not been able to find is 
> information on the physical size of  leases file and how much data the 
> DHCP service logging is going to add to the syslog (or wherever its put) 
> for a given address pool and lease time.  Does anyone have a link to that 
> information or a good estimate from their experience as to what I might 
> expect in the below environment?  Does going to a failover setup add a 
> noticeable percentage to either of these numbers?

Failover adds some additional timers to each lease.  It's not double,
but it is an increase.

> 60,000 available address
> 350 scopes
> 25,000 clients
> approximately 300,000 queries per day(average 4 queries per second)

Assume approx ~500 bytes per lease, with failover.  It varies, and it
depends on your enviroment and config.  Total size will look something
like:

	500 * (n + (c * (3600 / (t/2))))

n: number of leases
c: number of active clients.
t: lease-time, t/2 = renewal time.
3600: period at which the lease-file is rewritten (not configurable).

Plug in what values you supplied and we solve for:

	30000000 + (45000000000 / (t/2)))

	4 requests/s = 25000 clients / (t/2)

	t = 12500 (an hour and 20 minutes?  that's an odd choice)

	Est size: 37,200,000


In failover this gets a little more problematic because the 'initial
lease time' is MCLT.  It's impossible to guess how many of your
clients are going to be in the initial state, how many will be renewing,
and even how many will get 'MCLT optimal lease times' because their
potential-expiry is not yet up (so their initial lease time might be
somewhere between MCLT and lease-time).

It's probably reasonable to assume the worst case and substitute MCLT
for lease-time.


But: this is still an estimate.  If a 'bad' client out there is
renewing like mad (or churning through addresses due to some bug),
you'll use 500 bytes for each transaction with the client.  When
the disk fills and the write fails, you're toast unless removing
the dhcpd.leases~ temporary file frees up enough for a complete,
new file.


As for syslog, I generally just leave that up to log rotators.

-- 
ISC Training!  October 16-20, 2006, in the San Francisco Bay Area,
covering topics from DNS to DDNS & DHCP.  Email training at isc.org.
-- 
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