The first thing that I should say is that I am a big fan of tape. I have often been accused, by my other friends in the industry, of being anti-disk. However these new, really cheap, disk arrays have caused me to take a second look.
There are a number of ways that disk is better than tape:
1. It does not need to be streamed. This means that multiplexing, a feature of backup and recovery software that many administrators do not like, is not necessary. It's only necessary with tape because the tape must be streamed. Multiplexing allows multiple clients and multiple filesystems to be backed up to the same tape simultaneously, thus streaming the drive. However, this functionality slows down the restore. When backing up to disk, you do not need to multiplex.
2. Instant access. Most tape drives on the market have a "time to data" of close to two minutes. This time includes the amount of time to move the tape from its slot, load it into the drive and seek an appropriate place on tape. Disk has a time to data of effectively 0 seconds. You may wonder how important two minutes is during the restore. If you are restoring a large filesystem that happens to have its backups on 30 different tapes, then two minutes adds almost two hours to the restore. (Remember the time involved for the tape to be ejected and unloaded.)
3. No requirement for occasional full backup. The reason that we do occasional full backups with tape-based systems is the problem described above. If we don't do an occasional full backup, the number of tapes that are going to be required during a restore will significantly increase restore time and significantly increase the chances that a single tape will cause the restore to fail. Since these arrays are all protected by some level of RAID software and sends readings from 100 disks is faster than reading from one disk, these arrays do not suffer this problem.
I've always thought that disk would be a really neat medium for many types of backups but they were always too expensive. Well, now they're not.
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