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
