Kudos to all who contributed to MDB

[Brent Bice <bbice@sgi.com>]

   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