Backup Basics Part 2: Demystifying Backup Media
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Part 1 of this Backup Basics series covered the common types of backups included with most backup tools—as well as ways to support workstation backups in a network environment. This article focuses on the types of storage devices that you can use to actually store your backups. Like the previous article, it is written with server backups in mind, but it does also apply to individual computer backups.
In theory, you can store backups on any type of disk or media. However, for practical purposes, network backups are typically stored on tape or fixed media (hard drives or RAID arrays) or by using network storage technologies. You might also want to consider optical media (CDs or—increasingly more commonly—DVDs) for storing archived data or for data that does not need to be frequently changed, such as backups of workstation disk image files.
Tape
Digital tape is probably the oldest choice for backup media and it is still popular as an option today. Tape drives use magnetic tape (similar to that used by an audio or video cassette) to store data. The data is stored magnetically (as in a hard drive or a floppy), but the data is laid in sequential sectors along the length of the tape rather than across a platter or disc. This means that tape has a significantly lower cost per gigabyte of storage than hard drives. It also means that the tape drive must rewind and fast forward to different segments of a tape to retrieve data, making it much slower.
The slowdown due to locating data on a tape isn’t typically an issue when writing backup files to the tape because new data is usually appended to the end of the tape, with the tape’s catalog file (which is typically stored as a file separate from the tape), identifying which preexisting data has been superseded by the newer data as it is added. This leads to some wasted space on the tape (as older data is usually ignored rather than overwritten or erased) and it makes tape backups dependent on the catalog file. Most backup applications that support tape media can attempt to re-create a catalog file based on the data stored on the tape if the catalog file is lost or damaged, however.
Tape media cannot be interacted with directly from most operating systems (that is, if you put a tape into a tape drive, most computers won’t mount the tape as a volume allowing you to browse, open, or save files to it). A backup application is required to interact with the tape drive and the media, so your choice of drive type and media is limited to what is supported by your backup application of choice.
Tape is also physically more fragile than other media types. As with an audiocassette or VHS tape, if you open the cassette, you can pull the tape out and physically damage it. If exposed to significant heat, it will melt. And, like all magnetic media, the data on it can be damaged by exposure to strong electric or magnetic fields.
All this doesn’t mean that tape isn’t a viable option. It does, however, mean that you need to plan for the physical security of tape backups accordingly. It also means that maintaining multiple physical copies or sets of tape backups is often a good idea.
One of the big advantages of tape as a backup medium, beyond its cost, is that you can easily add tapes to a backup set as the amount of data you need to back up grows (for large organizations, there are tape autoloaders that can support backups that span multiple tapes). You can also create multiple sets of tape backup for fault tolerance. And tapes are extremely portable, making them a very easy choice for providing offsite storage of backup sets.
If you use tape media, you should plan to periodically recycle tapes and eventually replace them. Again, like audio and videotape, the quality of the data stored on a tape and eventually the quality of the tape itself will degrade over time. Because tapes store data digitally, this degradation isn’t as significant as with a VHS tape, but it can cause a problem over moderate to long periods of time (exact timeframes vary depending on the type and manufacturer of the tape). Recycling a tape is the process of erasing the tape and then using it for backup again. Periodically recycling tapes can improve their usable lifespan. You will also need to periodically clean the heads of a tape drive mechanism to ensure accurate function and to prolong the lifespan of individual tapes using a head cleaning tape. Recommendations on head cleaning and tape recycling or replacement will vary depending on the type of drive and media used.
Tape drive mechanisms, and the tapes that they use, come in a variety of formats. Some mechanisms offer increased performance; others offer greater reliability. Some offer higher performance at a cost of wear and tear on the tapes and the drive heads. You can find out more about specific tape drive types and models here.