If you use tape drives to store data for your archives or your backups, you probably have seen the error end of tape reached.
And you probably have no idea why you are reaching the end of a tape apparently much larger than the space occupied by the data you want to write on it.
Tape capacity is not an “exact measure” and may vary over time. Let’s see what affects it.
Tape capacity and data compression
There’s a lot of confusion regarding the advertised and effective capacities of 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 compressed: with these files there would be no gain and sometimes the compression would waste space instead of saving it. Furthermore different compression algorithms produce different results depending on the data.
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.
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 tape capacity.
Bottlenecks and/or misconfigurations of your hardware and network can increase the waste of space.
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.
What to do
First of all check the tape effective capacity. If you can’t push 580GB of data into a 400/800GB (real/compressed) LTO3 tape there’s no real problem here. You have to split the backup into multiple tapes.
Check the health of your drive and tape with a maintenance software like HP Library and Tape Tools. Then clean the read-write heads with the cleaning tape cartridge.
If the problem persists investigate the consistency of the data flow between the storage disks and the tape drive. Reducing the loads of the storage and network hardware can help containing the amount of data written.