The Files-11 file system uses a technique called caching to
improve performance. It keeps a copy of data that it has recently
read from disk or written to disk in an area of memory called a cache.
When an application reads data, the file system checks whether
the data is in its cache. It issues an I/O to read the data from
disk only if the data is not in the cache.
Caching improves read performance. Reading data from memory
(from the cache) is much faster than reading it from disk.
There are several levels of caching in both the hardware I/O
subsystem and in OpenVMS. In general, more levels of caching result
in better response time in accessing data.