General questions about failover, config changes and restarting

[sthaug at nethelp.no]

> > Sync is finished when both peers return to NORMAL mode. You need to
> > restart both servers (just kill dhcpd and restart it) one after
> > another or you're likely to run into issues with the pools not
> > matching, and then you'll run into issues with not leasing IPs.
...
> Any ideas why my servers need so many restarts? I try to leave them to settle for five or six minutes before trying again, but they just seem to stick with things like 

I am somewhat mystified about why you would need many minutes for a
restart. Here's what we see on our failover pair, with around 100k
leases and a couple of hundred pools:

(master restarts, log on slave):
Mar  3 08:50:00 slam dhcpd: peer dhcp1-dhcp2: disconnected
Mar  3 08:50:00 slam dhcpd: failover peer dhcp1-dhcp2: I move from normal to communications-interrupted
...
(a few seconds pass, and then)
Mar  3 08:50:11 slam dhcpd: failover peer dhcp1-dhcp2: peer moves from normal to normal
Mar  3 08:50:11 slam dhcpd: failover peer dhcp1-dhcp2: I move from communications-interrupted to normal

So a restart for us takes around 11 seconds.

It should be noted that
- The servers have plenty of memory, and hardware RAID with battery
backup for the disks.
- We use the "delayed ACK" facility.

Steinar Haug, Nethelp consulting, sthaug at nethelp.no