high performance open source DHCP solution?

Thu Jul 21 02:35:15 UTC 2011

On Jul 20, 2011, at 6:25 PM, Owen DeLong wrote:

> SSDs can be a good alternative these days as well. Some of them have gotten
> to be quite fast. Sure, you'll have to replace them more often than spinning media,
> but,

Actually the the scale of writes associated with this application is unlikely to significantly impact the service life of an SLC nand ssd with a solid block shadowing/wear leveling implementation. back in 2007 I was convinced that we could improve on the reliability of our network appliances with industrial 2.5" sata and enterprise sas disks, and the situation has only improved since.

> the write times can be quite a bit better.

like orders of magnitude.

> Owen
> 
> On Jul 20, 2011, at 3:28 PM, Jimmy Hess wrote:
> 
>> 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
>> 
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