Most critical to directory server performance is minimizing disk access. Defining a JVM heap size that can contain the entire contents of the database cache in memory is essential to minimizing read operations from disk and achieving optimal performance. It is also important to understand that the database on-disk is comprised of transaction log files, which are only appended to. After an initial database import, the size on-disk will grow by a factor of at least 25% as inactive records accumulate within the transaction logs. Therefore, during normal operation, the on-disk size of the database transaction logs do not represent the memory needed to cache the database.
Another consideration is to minimize the size of the database based on the known characteristics of your data. Minimizing the size of the database not only reduces hard disk requirements but also reduces the memory requirements for the database cache. The Directory Server has the capability to automatically compact common parent DNs, which is an example of optimizing the database size based on known characteristics of the data.
Another consideration is to consider the write load on your server and its affect on the database. While write operations will always require an associated write to disk, an environment that sustains a high load of write operations may consider tuning the background database cleaner to minimize the size of the database on disk.