7.7.3 Tape Backup
7.7.3.1 No single component of a digital system can be considered reliable, instead the reliability of the system is achieved through multiple redundant copies at every stage. The final and most important component in the storage chain is the data tape. In the recent past LTO has gained popularity for this purpose (see section 6.3.12 Selection and Monitoring of Data Tape Media), however other data tape formats may be appropriate depending on the particular circumstance.
7.7.3.2 All data on disk storage should be duplicated on a suitable storage tape. A minimum of two sets of data tapes must be produced, to be stored physically in different places. As it is not unusual for the second set of tapes to be required in the restoration of the data many established archives make three sets of copies, two to be kept near the system for ease of access and a third set stored remotely to protect against physical disasters. It has become customary that the separate sets of data tapes should be made using different products of which a considerable amount of the same batches are bought at one time. This renders quality control and rescue measures easier, once a batch of a given product should fail. Appropriate volume management software will aid in the back up and retrieval process especially if the system incorporates a number of storage devices.
7.7.3.3 Error checking is difficult to implement in open source and low tech solutions because that capability is linked to specific hardware. Nonetheless, a low-tech possible alternative to proper error testing is described in the following paragraph. The data management software has a catalogue (with a printer attached). The hard disc (in RAID) contains a complete set of data. All data is copied onto identical tape copies. There are at least two copies. As data is copied onto a tape, a unique identifier is printed onto a label (human readable) which is attached to the tape. The same identifier can be recorded onto the header of the tape. The data management system can be scripted to prompt the user to find and insert the tape identified by the system. Rather than checking the tape for errors, the system will verify the content of the tape against the hard disc. The hard disc can check the veracity of its own data content and is aware of any failings itself. If the verification of the tape fails, the system can produce a new tape from the hard disc. Assuming 20 terabytes of storage, the system would verify two tapes a day, every tape and its duplicate can be verified three times per year. In the event of a disc failure requiring the data tapes to replace it, there will be two tapes which have been checked within the previous four months. The risk that both tapes and the hard disc would fail is very low.
