RMS, Extended File Cache (XAB$ CACHE OPTIONS) (Alpha Only)
*Conan The Librarian
The ODS-2 and ODS-5 volumes of the Files-11 file system can use a caching technique to improve performance. In using caching, the file system keeps a copy of data that it recently read from disk in an area of memory called a cache. When an application reads data, for example, the file system checks whether the data is in its cache. The file system only issues an I/O to read the data from disk if the data is not in the cache. Caching improves read performance, because reading data from cache memory is much faster than reading it from disk. The extended file cache (XFC) is a virtual block cache, which caches both data and image files, and is available only on Alpha Systems. The extended file cache allows you to specify the following caching options: o Write-through caching o No caching You can control the files that the Extended File Cache option caches by setting and showing the current caching option. This is described in the following section. Setting and Showing the Current Caching Option When you access a file, you can specify the caching option that you would like for the current process. If you want the file to be cached, select write-through caching. This is the default. The write-through cache allows an application to write data to a file and straight through to disk. When this occurs, the application waits until the disk I/O is done and the data is on the disk. You can set the caching option by supplying a set mode XAB$_ CACHING_OPTIONS XABITM when you do the following: o When you create a file using SYS$CREATE o When you open an existing file using SYS$OPEN If you do not supply a XABITM or, if you supply a XABITM whose value is zero (0), the file system uses the value in the file's caching attribute. If another process on your computer is accessing the file, and you ask for write-through caching, your request is ignored if the file's current caching option is no caching. When more than one process is accessing a file on a single node, the most restrictive caching option takes effect on that node. Write-through caching is least restrictive; no caching is most restrictive. When more than one node in an OpenVMS Cluster is accessing a file, its caching option may be different on different nodes. It may be write-through on one node and no caching on another. To show the caching option, supply a sense mode XABITM on a call to SYS$DISPLAY.