Solving Problems with Corruption (SAP Library - SAP Database Guide: Informix (BC-DB-INF-DBA))

Solving Problems with Corruption

Use

This section tells you about the problem when Informix data or index pages are corrupted, that is, they are destroyed and can no longer be processed. This section covers how to:

Check for corruption so as to discover it as soon as possible
Deal with corruption once it has occurred

Caution

Give checking for corruption very high priority because the consequences of corruption going undetected for some time are serious. For more information, see SAP note 29155 and How to Run Data Consistency Checks with SAPDBA.

We strongly recommend you to check the entire database for corruption at least once per turnaround cycle of backup media (for example, tapes). Otherwise there is no way to restore the data in the event of corruption.

Procedure

Checking for Corruption

How you check for corruption depends on how much downtime you can afford on the production system and whether you have a test system available on a different machine:

Note

To check the entire database can take several hours, depending on the size of the database. During this time the system cannot be used productively because otherwise the results of the check are unreliable.

If you have enough downtime available, check the database for corruption at least once a week.
If you only have limited downtime available, you can reduce the downtime required for the check by fully exploiting parallelism. The maximum number of parallel processes depends on the maximum physical disk access that can run in parallel (that is, it depends on the number of controllers, disks, and so on).
If you have no downtime available on the production system, you can import the database backup onto a test system on a different machine, then run the check on the test machine. This has the following advantages:

It causes no downtime and no impact on the production system
At the same time you can check that the backup is OK

Caution

If you do not check for corruption regularly, you might run into serious problems with the backup. If you back up a corrupted database, the backup will also be corrupted. A corrupted backup can sometimes not be used for a restore. Or, if a corrupted backup is usable, it still contains the corruption, that is, destroyed data.

Therefore, the last method described above is the best.

Dealing with Corruption

Check your hardware. Corruption is very often caused by defective controllers, disks, and so on.

If necessary, repair or replace your hardware.

Decide how to recover the database to a consistent state, depending on the nature of the corruption:

If only index pages were corrupted, rebuild the indexes.
If necessary – for example, if a disk containing data was corrupted – perform a full database restore.

Note

Contact the Informix hotline in the following cases:

If you cannot deal with the corruption yourself
If the data is corrupt and you do not know whether the backup is also corrupt and whether it can be restored. It is likely that you will lose data in this case.

Result

You identify corruption as soon as possible after it has occurred. This means that you can react quickly and effectively, so keeping data loss and the associated costs to a minimum.