[EXTERNAL] Re: extremely slow DHCPD startup time

[Shane Merritt]

The server disks are already SSD.  The total max available lease space according to dhcpd-pools is 283,751.  I did a restart on one of the pair earlier and the time was about 10 minutes, but activity is low at the moment since fall semester hasn’t begun.

Shane Merritt
Office of Information Technology
The University of Alabama
smerritt at ua.edu


From: dhcp-users <dhcp-users-bounces at lists.isc.org> on behalf of Sten Carlsen <stenc at s-carlsen.dk>
Date: Thursday, August 4, 2022 at 2:54 PM
To: Users of ISC DHCP <dhcp-users at lists.isc.org>
Subject: [EXTERNAL] Re: extremely slow DHCPD startup time



On 4 Aug 2022, at 21.05, Greg Sloop <gregs at sloop.net<mailto:gregs at sloop.net>> <gregs at sloop.net<mailto:gregs at sloop.net>> wrote:

I'm sure others will have some useful suggestions - but I do know that improving disk I/O can seriously improve performance.
I'm not sure if that will improve restart/start/reload times.

It would be pretty easy to test though, say swapping for an SSD/Nvme.
(Or if you like living dangerously, putting the disk in RAM.)



On Thu, Aug 4, 2022 at 12:02 PM Shane Merritt <smerritt at ua.edu<mailto:smerritt at ua.edu>> wrote:
I have a failover pair of DHCP servers running 4.4.2-P1 that handle a DHCP for a very large wireless network (>75k devices active at any time). The leases file on these boxes is around 200 megabytes.  The problem I am having is when I need to make a change and restart, DHCPD takes over 25 minutes to start giving out addresses.  I assume this is due to the large lease file.  I see that there are tons of leases in the file that are “binding state free” that in some cases are months old.  Are there any suggestions to improve this situation?

AFAIK the server will fill its internal tables with every possible lease, most of them empty or marked free. I could be wrong about this.

So the question is how large your net blocks are?

If you e.g. have a /8 address space, that would amount to the server having to populate 2^24 (16 mill) entries in its internal table and do what ever processing etc it does for each entry. This may take time.




Shane Merritt
Office of Information Technology
The University of Alabama
smerritt at ua.edu<mailto:smerritt at ua.edu>

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