Summary
There are special restrictions for database (SQL and EXCH) restores and subsequently that introduces certain restrictions on their backups.
Resolution
Case 1:
For Example: If there is a base backup followed by multiple incremental backups, the base backup and all the incremental backups must be restored. And all those backups must be restored in the same order in which they were backed up.
Case 2:
Now consider the case where the user does one base backup and two incremental backups onto two tapes, tape A and tape B.
First he does a base backup of the pubs database of the MS SQL Server onto tape A. Then he does an incremental backup of pubs onto tape B. And after that he does an incremental backup of pubs onto tape A again, appending the data to the tape. What happens when he restores those backups?
In order for the restore to succeed, he must restore the first partition of tape A, the first partition of tape B and the second partition of tape A in that order. Unfortunately, in the current design, the data manager will sort the RJI file so that both partitions of tape A come before tape B. Because of that the data mover will restore the incremental backups in the wrong order and the MS SQL Server will issue an error claiming that the logs are not in sequence. This is avoided by placing the following restriction.
Tape Usage Restriction:
Either all incremental backups must go to the same tape one after another or if it transits to a different tape, it cannot transit back to the same tape again. A base backup has no such restrictions except that it uses a new/empty tape.
Please consider the following cases. The first two combinations either remains with the same tape or transits to different tapes and never attempts to use the old tapes it moved away from. The next two combinations (hypothetical) would cause problems because during the restore because we have the incremental logs out of sequence.
A Good Set of Tapes:
Base in Tape A
Incr1 in Tape A
Incr2 in Tape A
Incr3 in Tape A
Base in Tape A
Incr1 in Tape A
Incr2 in Tape B
Incr3 in Tape C
Incr4 in Tape C
A Bad Set of Tapes:
Base in Tape B
Incr1 in Tape A
Incr2 in Tape B
Incr3 in Tape A
Base in Tape A
Incr1 in Tape A
Incr2 in Tape B
Incr3 in Tape A
To reiterate the restrictions, please note that the transitions in italics (to a different tape not used before) are allowed based on tape availability. The transitions in bold (to a tape that was used for a base or incremental before and with at least one italic transition before the bold one) are always blocked by the data manager irrespective of whether tapes are appendable or not.