high performance open source DHCP solution?

Wed Jul 20 22:28:38 UTC 2011

On Wed, Jul 20, 2011 at 9:31 AM, Nick Colton <ncolton at allophone.net> wrote:
> We were seeing similar issues with low leases, moved the dhcpd.leases file
> to a ramdisk and went from ~200 leases per second to something like 8,000
> leases per second.

Yes, blame RFC2131's  requirement that a DHCP server is to ensure that any
lease is committed to persistent storage, strictly before a DHCP
server is allowed to
send the response to the request;   a fully compliant DHCP server with
sufficient traffic
is bound by the disk I/O rate  of underlying storage backing its database.

I do not recommend use of a RAMDISK;  it's safer to bend the rule than break it
entirely;   a safer way is probably to use a storage system on a battery-backed
NVRAM cache  that you configure to ignore SYNC() and lie to the DHCP server
application,  allowing the storage system to aggregate the I/O.

Of course,  committing to a RAMDISK tricks the DHCP server software.
The danger is that if your DHCP server suffers an untimely reboot, you
will have no transactionally safe record of the leases issued, when the
replacement comes up, or the  DHCP server completes its reboot cycle.

As a result, you can generate conflicting IP address assignments, unless you:
(a) Have an extremely short max lease duration  (which can increase
DHCP server load), or
(b) Have a policy of pinging before assigning an IP, which limits DHCP server
performance and is not fool proof.

--
-JH