I've started testing an LDAP server here using MDB and ran across a few caveats that might be of use to others looking into using it. But first off, let me say a hearty THANKS to anyone who's contributed to it. In this first OpenLDAP server I've converted over to MDB it's *dramatically* faster and it's definitely nice to not worry about having to setup script/s to occasionally (carefully) commit/flush DB logs, etc.
One caveat that might be worth mentioning in release notes somewhere... Not all implementations of memory mapped I/O are created equal. I ran into this a long time back when I wrote a multi-threaded quicksort program for a friend to had to sort text files bigger than 10 gigs and didn't want to wait for the unix sort command. :-) The program I banged together for him used memory mapped I/O and one of the things I found was that while Solaris would let me memory map a file bigger than I had physical or virtual memory for, linux wouldn't. It appeared that some versions of the 2.x kernels wouldn't let me memory-map a file bigger than the total *virtual* memory size, and I think MDB is running into the same limitation. On a SLES11 system, for instance with the 2.6.32.12 kernel, I can't specify a maxsize bigger than the total of my physical memory and swap space. So just something to keep in mind if you're using MDB on the 2.x kernels - you may need a big swap area even though the memory mapped I/O routines in the kernel seem to be smart enough to avoid swapping like mad.
On a newish ubuntu system with a 3.5 kernel this doesn't seem to be an issue - tell OpenLDAP to use whatever maxsize you want and it just works. :-)
I'd also only use MDB on a 64 bit linux system. One of the other headaches I remember running into with memory mapped I/O was adding in support for 64 bit I/O on 32 bit systems. Best to avoid that whole mess and just use a 64 bit OS in the first place.
Lastly... At the risk of making Howard and Quanah cringe... :-) The OpenLDAP DB I've been testing this with is the back-end to an email tracking tool I setup several years ago. More as an excuse to edjimicate myself on the java API for LDAP than anything else, I wrote a quick bit of java that watches postfix and sendmail logs and writes pertinent bits of info into an LDAP database, and a few PHP scripts to then query that database for things like to/from addresses, queue IDs, and message IDs. 'Makes it easy for junior admins to quickly search through gigabytes of logs to see what path an email took to get from point A to point B, who all received it (after it went through one or more list servers and a few aliases got de-ref'd, etc).
Yeah, it's an utter abuse of LDAP which is supposed to be write-rarely and read-mostly, especially as our postfix relays handle anywhere from 1 to 10 messages per second on average. :-) But what the heck, it works fine and was a fun weekend project. It's also served as a way to stress-test new versions of OpenLDAP before I deploy them elsewhere. :-)
Anyway, thanks again to everyone who contributed to MDB. It's lots faster than BerkeleyDB in all of my testing so far. 'Looking forward to gradually shifting more of my LDAP servers over to it.
Brent