How much write activity do you expect? How big entries? Will your entire DB fit in RAM, so disk read speed will mostly be an issue during startup (if back-bdb is properly tuned)?
Current stats for our slapd machines, which are currently vastly overpowered for their task:
- 3G database size from 0.5G LDIF data with 0.9 million entries.
- Average 60 connections and 280 requests per second the last two days, but just 6 per minute were write requests.
- 8G RAM - the entire databases can be cached in RAM.
- 4 2GHz cores in 2 processors, Linux x86_64.
- The 'unchecked' limit is set so that only the rootdn can trawl an entire DB with one search. Others get adminSizeExceeded if the number of candidate result entries after consulting the database indexes exceed the unchecked limit.
- RAID disks not specially tuned for BDB. Not recommended for speed nor database durability. But it's fast enough anyway and we can regenerate the DBs from LDIF, so we're happy to leave the disks to the site admins and the site's standard disk setup.
- The traffic gives 100K syslog output/second from loglevel 'stats'.
Some years ago, the disks on our aging machines got too slow for all the syslog output. Many requests took more than one second. So we had to turn off slapd logging. The issue went away when we put '-' in front of the logfile name in /etc/syslog.conf and upgraded to more modern hardware.
It would surely also have helped to play around more with the disks - like logging to separate disks if we did not already. Don't remember how much we did with that. We do like leaving such concerns to others at the site.