RevMedia

Mike Pope of Revelation Technologies reveals the following information courtesy of PT in the RevTech Labs.

The modulo of the file when it is opened is stored as a part of the file handle (See REVMEDIA Vol 2 Issue 7) in bytes 8 to 13. The modulos of five files are cached locally, bytes 2-7 of the handle are a pointer into the cache. The cached modulo is updated with each record I/O.

An implication is that file I/O to more than 5 files means cached modulos are discarded more often; on a network, the cache may not be maintained at all. If the modulo for the current file isn't in the cache, Linear Hash rereads the modulo from the file header - resulting in extra disk I/O and loss of performance.

The two points Mike makes from this are that 1) file I/O can suffer indirectly by accessing 6 files as part of a single process or on a network, and 2) the modulo in the handle isn't used for anything particularly useful.