The fields in the CacheInfo file are the only Cache Manager parameters that you must set. However, you can use the options available with the dfsd command to alter several Cache Manager defaults, affecting the way information is cached. Following are the configuration parameters that have the greatest effect on cache performance. (See the following subtopics for a description of the dfsd options used to configure these parameters.)
· Total Cache Size: This is the amount of disk space or memory available for caching.
· Chunk Size: This parameter determines the maximum amount of data that can fit in a cache chunk. A chunk cannot hold more than one element; in a memory cache, the unused memory that is allocated for a chunk is wasted. If an element cannot fit in a single chunk, it is split into as many chunks as are needed.
This parameter also determines the maximum amount of data that the Cache Manager can request at one time from a File Exporter. Increase the chunk size to take advantage of very fast links or decrease the size for slow networks.
· Cache Chunk Configuration: This parameter determines the number of chunks that are used for the cache. It can affect how often the Cache Manager must discard cached data to make room for new data. Consider the following example:
A disk cache is configured at 50 megabytes and consists of 1000 chunks. Suppose 10 users each have the Cache Manager cache 100 files and each file is 20 kilobytes in size. This uses all 1000 chunks available (because each chunk can hold only one element), even though the cache has only 20 megabytes of cached elements, which is less than 50% of its capacity of 50 megabytes.
When a user requests more data, the Cache Manager must discard cached data to reclaim space, even though the cache is not close to its capacity. In this case, increasing the number of chunks into which the cache is divided would improve performance by allowing the unused 30 megabytes of cache capacity to be allocated for other cached files.
· Number of dcache Entries in Memory: The Cache Manager maintains one dcache entry for each cache chunk; the entry records system identification information such as the file ID and version number of the file corresponding to the chunk. On disk caching machines, each dcache entry is stored in the CacheItems file, with a small number of entries (by default, 100) duplicated in machine memory. On memory caching machines, all dcache entries are stored in memory; the number of entries is equal to the number of chunks.
· Minimum and Initial RPC Authentication Levels: The Cache Manager maintains an initial authentication level used as a starting point for authentication negotiations with File Servers and a minimum acceptable level for communications with File Servers. There are two sets of these parameters: one set governs communications with File Servers in the local cell, and the other set governs communications with File Servers in foreign cells.
Note: The dfsd command is normally placed in a machine's initialization file (/etc/rc or its equivalent), not typed at the console. The commands in the following subtopics are presented as examples of entries from initialization files.
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