Read following instructions to avoid lack of free tapes during backup and adjust you backup strategy accordingly.
1. Select the minimum retention period
If you want to keep backups at least for 1 week, the retention period will be 1 week. Set this period for retention rules in the backup plans.
Example: I choose the retention period of 2 weeks.
2. Identify the maximum number of versions that can fit on the given number of tapes
Create 1 full, 1 differential or 1 incremental backup for each machine or backup plan depending on what type of backup you are planning to use.
Calculate how many versions of full backups or diff/incr backups will fill on the given number of tapes.
Example: I have 2 servers, SRV1 full is 150GB, SRV1 increment is 10 GB, SRV2 full is 230GB, SRV2 increment is 15Gb. I use LTO4 with 800GB storage size. It can store 800/(150+230)=2 full versions or 800/(10+15)=32 incremental versions on 1 tape. I have 4 tapes, so I can have 8 full or 128 incremental backup versions.
3. Choose the number of backup versions for the selected retention period
One retention period contains at least 1 full backup and may contain 1 or more diff/incr backups.
Example: I need to create 1 full backup weekly and 1 incremental backup daily. So 1 retention period will contain 2 full and 12 incremental backups.
4. Calculate the mutual size of all backups for the selected retention period
Mutual size is calculated for all machines or backup plan sources.
Example: 2 full and 12 incremental backups are 1060GB.
5. Calculate the number of tapes required for 1 retention period (retention tape set)
All backups for one retention period must fit on the selected number of tapes.
If an option Use a separate tape set for each single machine or Always use a free tape is selected, the minimum number of tapes will be equal to the number of full backups.
If you specify more than 1 tape drive in the vault and run parallel backup to that vault, number of tapes can be more than required for that amount of data since tapes are used simultaneously.
Example: I don't need to split backups to different tapes, so I will calculate the whole tape space size. For 1 retention period (1060GB) I need 2 tapes.
6. Calculate the available number of retention tape sets
Divide the number of all available tapes by the number of tapes in one retention tape set.
Example: I have 4 tapes, and 2 tapes are required for 1 retention period. So I have 2 retention tape sets.
7. Calculate the number of retention periods that can be placed on one retention tape set (fully or partly)
If backups from the first retention period do not fill the tape completely, backups from next retention period will be placed to the same tapes until they are fully written.
If an option Always use a free tape is selected For each full backup is enabled in all backup plans, the number of retention periods is always 1.
Example: 2 tapes from 1 retention set will contain 1060GB from the first retention period and 470GB from the second retention period. So the number is 2.
8. Evaluate the results
The sum of number of retention periods on one retention tape set and the number of retention tape sets must not be less than 5, and each of them must not be less than 2. Then the retention will be performed without problems with 1 vault and 1 pool setup.
In case both number of retention periods on one retention tape set and the number of retention tape sets are equal 2, divide the tapes in 2 pools equally, create 1 vault for each pool and 2 sets of backup plans. Backup plans' schedules should alternate each retention period.
In case there are tapes for less than 2 retention tape sets, consider reducing number of backup versions to increase the number of retention tape sets. If this is not possible, move tapes to Free tapes after a number of backups manually. The pool must also take tapes from Free tapes pool (pool settings).
Example: I have 2 retention tape sets and 2 retention periods can be placed on one retention tape set, so the sum is 4. This is the strategy I will apply. Backup plan 1 for 2 first weeks will backup to the first vault and will use 2 tapes from first pool. Backup plan 2 for 2 second weeks will backup to the second vault and will use 2 tapes from second pool.