Lease File of RAM Disk

[John Wobus]

> What should I do for the lease file in this case should I take a  
> backup of the lease file and place it on the new ramdisk and start  
> the server or should I start the server with an empty lease file.

We've played with this but don't run it in production.
Right now, we find using a disk controller with nonvolatile
writeback cache is nearly as fast and far more than adequate
for our needs.

If I were to use ramdisk, I'd copy the lease file to persistent
storage once a minute, and also adjust the dhcpd shutdown
script to do a final copy after dhcpd shuts down.

How important a copy is depends on whether you are running
redundantly and how long your leases are.  If all your
leases are 1 hour and you are down for 2 hours, starting
with an empty lease file costs nothing.  If you're not redundant
and are running 1 year leases, keeping the file could
be vital.  We're closer to the former so 60 seconds
of lost lease transactions probably won't be noticed,
especially in a large wireless environment where any
number of other issues are likely to cause a few
snits.

The DHCP protocol is designed to make no contribution to
random outages and conflicts: clients following the protocol
with a server following the won't create random conflicts
and outages.  With NV storage, it does this even with
server or OS crashes.  Your hardware and software can
have flaws but if there's a problem, it's the hardware
and software that need fixing: it's not some random
DHCP-generated issue.

With ramdisk you give up that strong assurance.  In
practical terms, in some site/DHCP setups, you're
giving up only a little, not enough to be noticed
in the noise.

John Wobus
Cornell IT