A logical-log backup is a copy to disk or tape of all full logical-log files. The logical-log files store a record of database server activity that occurs between backups.
To free full logical-log files, back them up. The database server reuses the freed logical-log files for recording new transactions. For a complete description of the logical log, see your IBM Informix: Administrator's Guide.
Even if you do not specify logging for databases or tables, you need to back up the logical logs because they contain administrative information such as checkpoint records and additions and deletions of chunks. When you back up these logical-log files, you can do warm restores even when you do not use logging for any of your databases.
A manual logical-log backup backs up all the full logical-log files and stops at the current logical-log file.
If you turn on continuous logical-log backup, the database server backs up each logical log automatically when it becomes full. If you turn off continuous logical-log backup, the logical-log files continue to fill. If all logical logs are filled, the database server hangs until the logs are backed up.
When the database server is offline (Dynamic Server) or in microkernel mode (Extended Parallel Server), you can perform a special kind of logical-log backup, called a log salvage. In a log salvage, the database server accesses the log files directly from disk. The log salvage backs up any logical logs that have not yet been backed up and are not corrupted or destroyed. The log salvage enables you to recover all of your data up to the last available and uncorrupted logical-log file and the last complete transaction.
Perform frequent logical-log backups for the following reasons:
Save the logical-log backups from the last two level-0 backups so that you can use them to complete a restore. If a level-0 backup is inaccessible or unusable, you can restore data from an older backup, if you have one. If any of the logical-log backups are also inaccessible or unusable, however, you cannot roll forward the transactions from those logical-log files or from any subsequent logical-log files.
To illustrate, as Figure 2 shows, suppose you perform a level-0 backup on Monday at 10:00 P.M. and then back up the logical logs on Tuesday at midnight. On Wednesday at 11:00 A.M., you suffer a mishap that destroys your databases. You would be unable to restore the transactions that occurred between midnight on Tuesday and 11:00 A.M. on Wednesday unless you had continuous logical-log backup set up.
If the disks that contain the storage spaces with the logical logs are damaged, the transactions after midnight on Tuesday might be lost. To restore these transactions from the last logical-log backup, try to salvage the logical logs before you repair or replace the bad disk and then perform a cold restore.
For more information, see Backing Up Logical Logs and Backing Up Logical-Log Files with ontape.
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