TurboIMAGE/XL Database Management System Reference Manual MPE/iX V6.5 (30391-90011)

Appendix G 663
Recovery and Logging Quick Reference
Logging Device Quick Reference
Logging Device Quick Reference
The lists below outline both the benefits and disadvantages of logging to disk and logging
to tape. Refer to chapter 7, "Logging and Recovery" for more information on user logging.
You must determine which type of logging device to use based on equipment, operations
staff, number of users and size of the database, and other considerations listed here.
Logging to Tape
• For roll-back recovery, requires the database be on the system volume set.
• Does not take up disk space.
• Requires a dedicated tape drive.
• Requires a reliable tape drive and a library of "good" tapes.
• Is more secure in terms of a hard crash.
• Is more time consuming. After a system failure the tape must be rewound and
sequentially scanned until the end of file is detected. The remaining records are then
appended to the file.
• The console operator must be available to respond to requests for tape mounts. If a
request is ignored and you run out of memory buffer space, logging will suspend.
Applications requiring logging will get a WRITELOG error and will terminate.
• Must do a start recovery operation after a soft system crash to write a crash log record
to the log file.
• During a start recovery operation, the console operator can respond with an option to
override or cancel the cleanup procedure on log files. Fewer log records written just
prior to system failure are lost.
• Does not provide any security measures to prevent overwriting the current tape. The
console operator should use care to mount a new tape before placing on-line. Read the
labels on the MPE/iX tapes for overwrite security information.
• Has overhead similar to logging to disk.
Logging to Disk
• For roll-back recovery, requires the database and the user log file to be on the same
volume set.
• Files are susceptible to a hard crash.
• The integrity of the log file may be no better than the current database state. The log
file may contain inconsistencies, bad characters, or other invalid data.
• If you are not using the GETLOG command AUTO option or the CHANGELOG command,
you must make sure disk file space of the current log file is sufficient so that end of file
is not reached. If the end of file is reached, logging will stop. Applications requiring
logging will get a WRITELOG error and will terminate.