iX V6.5 (30391-90011)

364 Chapter7

Logging and Recovery

The Mirror Database

Transaction logging and regular backups are good maintenance. However, if databases

must be accessible at all times and cannot be down even for maintenance, then a different

maintenance method is needed. A system can be set up for constant access or "high

availability," and still have controlled maintenance.

The mirror database is the fundamental element in creating a high availability database

system. This system consists of two identical databases on two separate computer systems.

One database is housed on a primary system and is constantly accessible to users and

application programs. The other "mirror" database resides on the secondary system and is

used for maintenance.

To establish a mirror database, the following requirements are necessary:

• Two systems--one primary, one secondary.

• Two identical copies of the database(s) are needed, one copy on the primary system, one

on the secondary system.

• All transactions on the primary system must be logged to a permanent ﬁle.

• Periodically, the log ﬁle containing the transactions must be moved or copied to the

secondary system, and used to update the database(s) on the secondary system. It is

recommended that the log ﬁle be kept on a private volume separate from the database or

on magnetic tape.

After the secondary system is established, it can be used to make backups of the database.

The primary system never has to be brought down for maintenance.

Transferring Log Files

The GETLOG command with the AUTO option and the CHANGELOG command provide the

capability to schedule secondary system backups through various methods of logging.

Listed below are four ways of copying log ﬁles from the primary to the secondary system.

The method chosen should depend on the maintenance needs.

1. Copying ﬁles over a direct DSLINE from the primary to the secondary system.

2. Logging to a private volume, entering the CHANGELOG command to start a new log ﬁle,

copying the closed log ﬁle to another private volume, and physically transporting that

second private volume to the secondary system.

3. Logging to disk, entering the CHANGELOG command to start a new log ﬁle, copying the

closed log ﬁle from disk to tape, and restoring the log ﬁle from tape to a disk on the

secondary system.

4. Logging directly to tape and mounting the tape on the secondary system.

Figure 7-3. illustrates the four methods of copying log ﬁles as listed above.