QTIPS - File Handle Structure

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.

(Volume 3, Issue 7, Page 7)