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.
The filesystem cache can be used as a replacement for some of the database cache. Database cache is more efficient for Directory Server use than file system cache, but file system cache is not directly associated with the Directory Server ns-slapd process. Potentially, a larger total cache can be made available to Directory Server than would be available by using database cache alone.
Refer to the operating system documentation for information about file system cache.