IBM i OS Journaling and High Availability

IBM’s midrange computers, from the earliest System/38 to today’s Power Systems servers, have a reputation for reliability. That reliability is in great part due to the journaling features that have been present in all variations of its operating system. Every system event and configuration change is journaled (logged) so that recovery from application errors or hardware problems is easier and faster.

Journaling was initially introduced as a feature of the operating system on IBM System/38 computers and was intended to aid technicians in their system recovery efforts. In the event of a catastrophic failure, the system administrator could reload the last good backup tape and, by applying the saved journal entries that had accumulated since that backup tape was made, restore the database to the point in time when the journal entries were last saved to tape. Less catastrophic system crashes, in which there was only the loss of data in memory, were an easier recovery. At the power-up Initial Program Load (IPL) following the crash, the journal entries stored on disk would be sequentially applied to the database, recovering the lost transactions up to the point of failure.

As one would expect, the necessary recovery process for system failures was a time-consuming operation. Retrieving the backup tapes from their offsite location and first loading the system tapes and then the journal entry tapes to recover a large and complex database took several hours at best and sometimes several days. Organizations with a low tolerance for extended downtime increasingly found the tape-based recovery method inadequate.

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