Save logical-log backups

You should perform frequent logical-log backups and then save the logical-log backups from at least the last two level-0 backups so that you can use them to complete a restore.

Perform frequent logical-log backups for the following reasons:
  • To free full logical-log files
  • To minimize data loss if a disk that contains logical logs fails
  • To ensure that restores contain consistent and the latest transactions
You should save the logical-log backups from the last two level-0 backups because if a level-0 backup is inaccessible or unusable, you can restore data from an older backup. 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.
Important: You lose transactions in logical-log files that are not backed up or salvaged.

To illustrate, as the following figure shows, suppose you perform a level-0 backup on Monday at 10 p.m. and then back up the logical logs on Tuesday at midnight. On Wednesday at 11 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 a.m. on Wednesday unless you had continuous logical-log backup setup.

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.
Figure 1. Storage space and logical-log backups
This figure is described in the surrounding text.