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In years past, disk drives would never have been used as a backup medium. However, storage prices have dropped to the point where, in some cases, using disk drives for backup storage does make sense.
The primary reason for using disk drives as a backup medium would be speed. There is no faster mass storage medium available. Speed can be a critical factor when your data center's backup window is short, and the amount of data to be backed up is large.
But disk storage is not the ideal backup medium, for a number of reasons:
- Disk drives are not normally removable. One key factor to an effective backup strategy is to get the backups out of your data center and into off-site storage of some sort. A backup of your production database sitting on a disk drive two feet away from the database itself is not a backup; it is a copy. And copies are not very useful should the data center and its contents (including your copies) be damaged or destroyed by some unfortunate set of circumstances.
- Disk drives are expensive (at least compared to other backup media). There may be situations where money truly is no object, but in all other circumstances, the expenses associated with using disk drives for backup mean that the number of backup copies must be kept low to keep the overall cost of backups low. Fewer backup copies mean less redundancy should a backup not be readable for some reason.
- Disk drives are fragile. Even if you spend the extra money for removable disk drives, their fragility can be a problem. If you drop a disk drive, you have lost your backup. It is possible to purchase specialized cases that can reduce (but not entirely eliminate) this hazard, but that makes an already-expensive proposition even more so.
- Disk drives are not archival media. Even assuming you are able to overcome all the other problems associated with performing backups onto disk drives, you should consider the following. Most organizations have various legal requirements for keeping records available for certain lengths of time. The chance of getting usable data from a 20-year-old tape is much greater than the chance of getting usable data from a 20-year-old disk drive. For instance, would you still have the hardware necessary to connect it to your system? Another thing to consider is that a disk drive is much more complex than a tape cartridge. When a 20-year-old motor spins a 20-year-old disk platter, causing 20-year-old read/write heads to fly over the platter surface, what are the chances that all these components will work flawlessly after sitting idle for 20 years?
NoteSome data centers back up to disk drives and then, when the backups have been completed, the backups are written out to tape for archival purposes. This allows for the fastest possible backups during the backup window. Writing the backups to tape can then take place during the remainder of the business day; as long as the "taping" finishes before the next day's backups are done, time is not an issue.
All this said, there are still some instances where backing up to disk drives might make sense. In the next section we see how they can be combined with a network to form a viable (if expensive) backup solution.