I have a ZNE 5.0.9 server running on a CentOS 5.2 box, x86_64. The server has 2 dual-core 2.8GHz Opteron processors, and 8GB of memory. Mail is stored on an external array with 12x146GB 15k disks running hardware RAID 5 and connected via SAS. The OS and Zimbra installation directory are on the server's internal disks (2 146GB 10k RPM SAS disks, hardware RAID 1). My zimbra configuration is reasonably vanilla (no exotic stuff) except for an external LDAP authentication server (user accounts are stored in the Zimbra LDAP, but auth goes to an external server). I currently have about 400 users on my system, only about 100 active at a time. Performance is generally good, until this morning, when IMAP login attempts became intermittently slow. I attempted a few diagnostic connections with OpenSSL, and I show that I get a big (>10s) lag after connecting to the server before getting the IMAP banner. Specifically, the lag happens after the connection is opened but before the SSL certificate is presented.

While all this is going on, the system looks good from all of the views that I know of -- vmstat reports memory usage is good and no swapping going on, iostat says the drives aren't being hammered and have reasonable svctm values, top doesn't show anything out of the ordinary, load on the box is usually around 1.0-1.5, and the only other service the server provides is a SpamAssassin spamd that sees typically 3 messages per minute. I don't run either the Zimbra spam or virus filtering, and my mail rate is about 1000 messages per hour at peak.

Has anyone else had this problem? Any suggestions for which logs to look at or any knobs to turn?