HP 3000 Manuals HP 3000 Manuals
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.
* Must do a start recovery operation after a soft system crash to
write a crash log record to the log file.
* Has overhead comparable to logging to tape.
NOTE In the event of a system failure and subsequent start recovery
operation and when using private volumes, logging will not resume
until these private volumes have been mounted. Enter the MPE
VMOUNT command into the SYSSTART file to resume logging.