On Tue, Jul 21, 2009 at 01:54:25PM -0700, Quanah Gibson-Mount wrote:
--On Tuesday, July 21, 2009 4:51 PM -0400 "Clowser, Jeff" jeff_clowser@fanniemae.com wrote:
Do you have any facts/numbers to back this up? I've never seen F5's slow things down noticably.
We've had F5's be the root of the problem with several clients who load balanced their LDAP servers, and pointed postfix at the F5 for delivery. They added just a few milliseconds of time to each LDAP query, but that was enough to completely back up their mail delivery system. <...>
Given the reported log message, this (latency) is very likely to be the cause of the problem. "connection deferred: binding" means that the server received a request on a connection that was in the middle of processing a bind. This means that the client sends a bind and then additional request(s) without waiting for the bind result. That's a violation by the client of the LDAP protocol specification, RFC 4511, section 4.2.1, paragraph 2:
After sending a BindRequest, clients MUST NOT send further LDAP PDUs until receiving the BindResponse. Similarly, servers SHOULD NOT process or respond to requests received while processing a BindRequest.
The log message is slapd saying "I'm obeying that SHOULD NOT for this connection, loser". It should be obvious now why the conn_max_pending* options have no effect.
Understanding _why_ clients are violating the spec by sending further requests while a bind is outstanding may help you understand how the F5 or the clients should be tuned (or beaten with sticks, etc).
You presumably don't notice this under normal circumstances or with RR DNS because the server completes the BIND before the next request is received. My understanding (perhaps suspect) is that the F5 will increase the 'bunching' of packets on individual connections (because the first packet after a pause will see a higher latency than the succeeding packets).
So, are you measuring latency through the F5? I would *strongly* suggest doing so *before* tuning the F5 in any way, such as by the VIP type mentioned by John Morrissey, so that you can wave that in front of management (and under the nose of the F5 saleman when negotiating your next support renewal...)
Philip Guenther