Extended_File_Cache_(XAB$_CACHE_OPTIONS)_(Alpha_Only)

   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.