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Plane or train? Tape or disk? Reg readers speak

Disk speed versus tape economy and removabiity

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Oculus - IT Specialist

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If you use tape it has to me managed; simple, no? Oculus writes about a situation where tape was used to back up sensitive data but not managed at all.

A project involved sensitive data. The sensitivity required that the data should be held on a server located within the project office, under suitable access controls and physical protection (which included the requirement that the server should be powered down and the disks moved into a safe at the end of the working day). Initially, the project was small but rapidly growing; the data was held on four hard disks arranged as two RAID-1 pairs (the disks are mirrored in pairs so that no data is lost in the event of any single disk failure).

Then the project became much larger, and the server was moved into its own “cage”. It had also been expanded, and a daily backup-to-tape instituted (the tapes were held in secure storage, and a rotation scheme was used so that at any time there were several daily and several weekly backups held on different tapes). As the backup took several hours to complete, it was run overnight (the “cage” affording sufficient physical security to allow the server to be run continuously), and the task of starting the backup run was assigned to any of several junior staff.

The backup routine persisted for several months without incident. But one morning someone took a closer-than-usual look at the console in the morning, and found that the backup run had finished with an error condition: it had run out of tape. Checking the other tapes in the collection, the team found that every backup in the collection was incomplete: they had all run out of tape, and nobody had noticed. For at least a few weeks, and probably for several months, there had been no serviceable backups. Fortunately, there had never been occasion to attempt to restore data from the backups.

Lessons learned: Firstly, ensure that you are capable of restoring from your backups. Secondly, ensure that you check for error messages! Thirdly, make sure that any junior staff assigned to routine tasks are able to respond appropriately (e.g. by getting a more senior colleague to take a look) if anything unexpected happens.

Oculus is the pseudonym of someone who is not a specialist in storage; he is a specialist in another field, who has been around long enough to see how storage works – and how it fails. A feature of the project mentioned is that there was no option to use an external data centre, because the project used data which was too sensitive to trust to an outside agency. It was not appropriate to name names, projects or the firm.


Summary

The consensus is that tape drives will predominate in archive-focused data protection applications, with disk drives being preferred for backup apps; no surprises really, unless you are of the disk-can-do-it-all persuasion. From the archive point of view tape-based storage is much more cost-efficient, removable for off-site data protection and fast enough for the purpose.

Of course it would be good if it were faster still and, thankfully, newer tape formats transfer data faster than old ones, and coming tape formats, such as LTO7, will transfer data faster still. If the disk-based backup array vendors manage to get the cost of a disk archive down to the tape area then that would erode tape's popularity as an archive medium but it would still have its removability advantage. ®

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