Each instance of Directory Server has one database cache. The database cache holds pages from the databases that contain indexes and entries. Each page is not an entry, but a slice of memory that contains a portion of the database.
The entry cache holds recently accessed entries that are formatted for delivery to client applications. The entry cache is allocated as required until it reaches a size larger than, but based on the maximum entry cache size you specify.
The import cache is created and used when a suffix is initialized. If the deployment involves offline suffix initialization only, import cache and database cache are not used together. In this case, the import cache and database cache do not need to be added together when the cache size is aggregated. See Total Aggregate Cache Size. When the import cache size is changed, the change takes effect the next time the suffix is reset and initialized. The import cache is allocated for the initialization, then released after the initialization.
The operating system allocates available memory not used by Directory Server caches and other applications to the file system cache. The file system cache holds data that was recently read from the disk, making it possible for subsequent requests to obtain data from cache rather than having to read it again from the disk. Because memory access is many times faster than disk access, leaving some physical memory available for the file system cache can boost performance.