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Plane or train? Tape or disk? Reg readers speak

Disk speed versus tape economy and removabiity

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You the expert Plane or train? We asked four Reg-readers with storage smarts to say where and when we should use disk-based data protection and where we should cross the line and use tape. Three did just that. The fourth identified a fourth use-case for tape and added a salutary reminder that it has to be managed; it is absolutely not a start-backup-run-and-forget option. The consensus was that neither disk nor tape on their own are sufficient.

Disk is in because its faster to backup to disk and restore from it, but tape is not out, not at all IT has substantial cost advantages, holding much more data for less money, and it can be stored off-line, even off-premise, making it a better insurance against disaster striking a data centre. In general disk has not replaced tape, and probably won't.

H Wertz - Freelance system administrator

I think the availability of disk-to-disk backup and virtual tape library (VTL) systems has reduced the need for tape, but tape still has an important role for archival purposes.

There are uses where a disk-to-disk backup or a VTL excels. One of these is in cases where frequent retrievals are expected, such as users frequently deleting or overwriting files. (On a side note here, VMS had/has a versioning file system, which would inherently keep older copies of files available for easy retrieval, and just remove the oldest ones as the disk filled up. But this is a very unusual feature. I have not heard of another system that has this.)  When I was a student in the late 90s, it'd take me about 30 minutes to pull a file off the departmental DDS-3, and it would take closer to 4 hours (and a hefty fee) for ITS to pull a file off the backup of main university systems.

Retrieval from a VTL would have taken a minute or two tops to find and retrieve a file. The big disadvantages of VTL? Since it is really an array of disks, software faults, hardware faults, or administrative faults could all render the library useless. In addition, the VTL would ordinarily be on-site.

Tape is still quite important for archival purposes, both for compliance and especially for disaster recovery purposes. A tape is written, then packed away and stored, so, unless tapes are re-used, those tapes provide an immutable record of what is on the system up to that point. Once it's ejected it won't be accidentally overwritten, erased, or modified. In addition, the tapes can be stored off-site, so in case of disaster the tapes won't be destroyed as the VTL could be. There are a few disadvantages, primarily "bit rot", the obvious retrieval speed disadvantage, and the cost of moving and storing tapes.

There are several technologies that could reduce the role of tape. First, internet backup allows for off-site storage without having to physically transport anything. However, like VTL it could allow for backups to be modified or deleted. Additionally, if you're using a service provider (instead of your own second site), it'd be a very good idea to verify if they have a robust set-up. One or two in the last 10 years have had a single hardware failure knock them off the face of the earth.  Bandwidth is also a big issue: how long will it take to restore the entire system?

Second, systems that use removable hard disks. These provide the advantages of tape (disk can't be accidentally modified if it's not plugged in, can be stored off-site, and so on. In addition, it speeds up file retrieval; someone still has to insert the right disk, but then retrieval is nearly instant.

The disadvantages? Potentially reliability (although I've had great luck hooking up disks that have sat doing nothing for years). The big one? Price; tapes cost about 1/10th the cost per byte of hard disks.

Mainframes are a special case. In terms of disaster recovery, they have supported synchronisation of services and storage between multiple sites for over 10 years, so in case of a disaster a backup system can be ready to go. Also, mainframes have extensive journaling, so in case of a problem the system can be rolled back to an earlier state. However, if the goal is to eliminate the potentially thousands of tapes, a VTL is also required, as mainframe software and procedures assume at least one tape drive.

In conclusion, tapes may not have the high profile they used to, but they are still important for archival and disaster recovery purposes.

Henry Wertz graduated from the University of Iowa in 2000. He has been a Linux user since 1994 (loading off floppies makes a CD install seem like luxury!), and into cars (both fast ones, and ones that are highly efficient). He is currently doing freelance computer work. He is also a regular commenter on Reg stories.

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