Types of Cache

This section describes the types of cache used by Directory Server.

Figure 5–1 shows the caches for an instance of Directory Server with three suffixes, each with its own entry cache.

Directory Server also uses a file system cache. The file system cache is managed by the underlying operating system, and by I/O buffers in disk subsystems.

Figure 5–1 Entry and Database Caches in Context

Database Cache

Each instance of Directory Server has one database cache. The database cache holds pages from the database that contain indexes and entries. Each page is not an entry, but a slice of memory that contains a portion of the database.

Directory Server moves pages between the database files and the database cache to maintain the maximum database cache size you specify. The amount of memory used by Directory Server for the database cache can be larger than the specified size. This is because Directory Server requires additional memory to manage the database cache.

For very large database caches, it is important that the memory used by Directory Server does not exceed the size of available physical memory. If the available physical memory is exceeded, the system pages repeatedly and performance is degraded.

The memory can be monitored by empirical testing and by the use of tools such as pmap(1) on Solaris systems. The ps(1) utility can also be used with the -p pid and -o format options to view current memory used by a particular process such as Directory Server ns-slapd. For more information, refer to the operating system documentation.

For 32-bit servers, the database cache size must be limited so that the total Directory Server ns-slapd process size is less than the maximum process size allowed by the operating system. Usually, this limit is in the 2-3 GB range.

Entry Cache

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.

As entries stored in the entry cache are already formatted, Directory Server returns entries from an entry cache efficiently. Entries in the database must be formatted and stored in the entry cache before they are delivered to client applications.

The maximum size you specify indicates how much memory Directory Server requests from the underlying memory allocation library. Depending on how the memory allocation library handles requests for memory, the actual memory used may be much larger than the amount of memory available to Directory Server for the entry cache.

The memory used by the Directory Server process depends on the memory allocation library that is used, and depends on the entries cached. Entries with many small attribute values usually require more overhead than entries with few large attribute values.

For 32-bit servers, the entry cache size must be limited so that the total Directory Server ns-slapd process size is less than the maximum process size allowed by the operating system. In practice, this limit is generally in the 2-3 GB range.

Import Cache

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.

Directory Server handles import cache in the same way as it handles database cache. Sufficient physical memory must be available to prevent swapping. The benefits of having a larger import cache diminish for cache sizes larger than 2 GB.

File System Cache

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.

For 32-bit servers, a file system 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.

64-bit servers do not have the same process size limit issue as 32-bit servers. Use database cache instead of file system cache with 64-bit servers.

Refer to the operating system documentation for information about file system cache.

Total Aggregate Cache Size

The sum of all caches used simultaneously must remain smaller than the total size of available physical memory, minus the memory intended for file system cache, minus the memory intended for other processes such as Directory Server itself.

For 32-bit servers, the total aggregate cache size must be limited so that the total Directory Server ns-slapd process size is less than the maximum process size allowed by the operating system. In practice, this limit is generally in the 2-3 GB range.

If suffixes are initialized while Directory Server is online, the sum of the database cache, the entry cache, and the import cache sizes should remain smaller than the total size of available physical memory.

Table 5–1 Import Operations and Cache Use

Cache Type	Offline Import	Online Import
Database	no	yes
Entry	yes	yes
Import	yes	yes

If all suffixes are initialized while Directory Server is offline, the import cache does not coexist with the database cache, so the same memory can be allocated to the import cache for offline suffix initialization and to the database cache for online use. If you opt to implement this special case, however, ensure that no online bulk loads are performed on a production server. The sum of the caches used simultaneously must remain smaller than the total size of available physical memory.