For ODS-2 and ODS-5 volumes, the Files-11 file system has
several caches. It has metadata caches for file metadata such
as file headers, and a data cache for file data. It can use
one of two system-wide data caches, as follows:
File System Data Cache
Description
Virtual I/O
Cache (VIOC)
This is the original data
cache, and is available on VAX, Alpha, and I64.
Extended File Cache (XFC)
This data cache, which is available on
OpenVMS Alpha Version 7.3 and later and on I64 systems, is available
only on OpenVMS Alpha and I64 systems. It provides better performance
and more capability than VIOC. XFC is the default cache on OpenVMS
Alpha Version 7.3 and later and on I64.
This chapter describes how to manage the data caches. For
information on managing the metadata caches, see OpenVMS Performance Management. Note
that RMS makes use of local and global buffers that can perform
data caching. By default, this caching is not enabled. You can manipulate
the RMS local and global buffers to affect I/O performance. For
information on managing the RMS local and global buffers, see the Guide to OpenVMS File Applications.
Note also that the modified page list is a type of cache; that is,
if a process references a page that is on the modified page list,
the page is placed back into the working set of the process and
is not output to disk.
Both XFC and VIOC are virtual block caches that maintain consistent
data within an OpenVMS Cluster. They cache both data files and image
files. The
data caches are write-through caches; when an application writes
data to a file, the data is written straight through to disk. The
application has to wait until the disk I/O completes and the data
is on disk.
In an OpenVMS Cluster, different nodes can use different data
caches. This allows mixed architecture clusters to benefit from
XFC. OpenVMS Alpha and I64 nodes can use either XFC or VIOC. OpenVMS
VAX nodes can use only VIOC, as described in
Using XFC in a Mixed Architecture OpenVMS Cluster.
XFC improves I/O performance and contains the following features
that are not available with VIOC:
Read-ahead caching
Automatic resizing of the cache
Larger maximum cache size
No limit on the number of closed files that can
be cached
Control over the maximum size of I/O that can be
cached
Control over whether cache memory is static or dynamic