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Understanding File System Data Caches  



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:


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