There’s a lot of confusion regarding the advertised and effective capacities of the tapes. Usually the cartridge box shows the maximum capacity achievable through a compression ratio of 2:1. In a nutshell, it means manufacturers assume that 800GB of data can fit into a 400GB – real capacity – tape.
This scenario is more than optimistic and we have to consider that most of today’s data formats – like .docx, .png or .jpeg – are already compressed: with these files there would be no gain and sometimes the compression will waste space instead of saving it. Furthermore different compression algoritms produce different results depending on the data copied.
A far more reasonable “optimal tape capacity” stays in a range between the physical one, 400GB for a standard LTO3 tape for instance, and a low compression ratio like 1.2:1 or 1.3:1 (if compression is enabled).
There are other variables that influence the maximum tape capacity.
Data flow from the SCSI host to the tape drive
In order to efficiently write on the cartridges, tape units need to receive a consistent and fast-enough data flow. The drives write data in big chunks called frames and when the flow is interrupted they fill them with zeroes, wasting the tapes capacity.
Bottlenecks and/or misconfigurations of your hardware and network can amplificate the wasted space on your tape.
When the tape drive detects write errors it has to re-write the data. Old tapes, dirty read-write heads or a faulty drive can waste most of the available capacity. It’s important to regularly clean the drive heads in order to maintain the optimal capacity.
Maintaining the performance</4>
Tape drives are complex solutions and need some cares in order to maintain their performance. You have to clean the heads periodically and it’s important to not load the network and the storage system with heavy tasks during the backup operations.
Finally, remember that tape cartridges need to be checked regularly and a backup without a restore test can be a dangerous bet.
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