Choosing Continuous Logical-Log Backup (ON-Archive) (SAP Library - SAP Database Guide: Informix (BC-DB-INF-DBA))

Choosing Continuous Logical-Log Backup (ON-Archive)

Use

You can back up the logical log of your Informix database using continuous backup with ON-Archive . With continuous backup, you start a process that backs up the individual logical-log files as they fill, either to tape or disk. SAP recommends that you back up to tape if possible because the process is easier. Continuous logical-log backup is often better than automatic backup (especially if you are only doing a periodic automatic backup, for example, once a day). Continuous backup for ON-Archive has been improved with Informix version 7.2.

Note

If your Informix version is older than 7.2, SAP recommends you not to use continuous backup since it is difficult to operate. Starting with version 7.2, improvements have been made making it easier to use. The information in this section relates to the improved backup.

Prerequisites

To use continuous logical-log backup, you need to define volume sets and volumes. Refer to Setting Up Volume Sets and Volumes for Logical-Log Backup (ON-Archive).

Procedure

Decide whether to use continuous or automatic logical-log backup.

The following table summarizes the advantages and disadvantages of continuous backup:

Summary of Continuous Logical-Log Backup

Advantages	Disadvantages
The logical log is constantly backed up so chance of full logical log should be reduced. If a disk staging area is used, there is no danger that an incorrectly mounted tape halts the backup. The worst-case scenario – logical log and its mirror both fail – only leads to loss of part of one logical-log file.	Operator is required to ensure that disk backups (if used) are copied to tape in time (an extra step). Informix offers no way to guarantee that the backup process is running when the system is started. Overall, the chances of full logical log probably not reduced compared to "scheduled and triggered" automatic backup.

The remainder of this process assumes that you choose continuous logical-log backup.

Decide which of the following approaches to use:

Back up directly to tape.

You can use either attended or unattended mode:

Attended mode

When the current tape volume is full, you get a message. You then interrupt the continuous backup request, change the tape, and continue with a new backup request. Each tape volume contains one save set.

Unattended mode

SAP recommends that you do not use this method since it is difficult to see when the tape volume is full.

Back up to a temporary storage area on disk, then copy to tape later.

With this approach, you back up to disk in unattended mode and later copy to tape in attended mode. This approach causes problems when you copy the disk backup to tape:

You must still make sure that the copy to tape is performed in time (so you can delete the disk backup). Otherwise your disk fills up and you have a full logical log.
The copy to tape usually takes a long time, which means that disk space is used and the chances of a full logical log are increased.
Although the logical log is only backed up to disk, it is still vulnerable to a system failure when several disk drives fail. Note that the logical-log backups ought to reside on a different disk to the disks with the logical log and its mirror.

Result

You have decided to use continuous logical-log backup and you have decided which type to use. This is an essential part of deciding your approach to logical-log backup with ON-Archive .

See also:

Informix documentation at http://www.informix.com