Pros and Cons of Removable Disks

Instead of using tapes, many people use removable disks as backup media. Table 2.5 in Chapter 2 summarizes many of the available removable disk technologies. At the low end, common 3.5-inch floppy disks hold 1,44MB. At the high end, special mounts are available to turn ordinary hard disks into removable media. You can also buy external SCSI, USB, or IEEE-1394 hard disks.

One of the prime advantages of removable disks—especially removable hard disks—is their speed. Modern hard disks can easily exceed 20MB/s; manufacturers typically claim speeds of 30-75MB/S, although actual speeds are likely to be substantially less than this. Unlike tapes, disks permit random access to data, which can greatly speed data recovery. By copying files using commands such as cp, you can use a disk for backup much as you might use a fixed hard disk. For backup purposes, though, using tar or some other backup tool may be preferable because these tools can do a better job of preserving all the important characteristics of a file—its ownership, permissions, status as a symbolic link, and so on. You can also use the backup software's compression scheme to fit backups from a larger disk on a smaller medium. Depending on how you use the backup software, though, using it may deprive you of the random-access nature of the disk technology.

Over the past few years, hard disk prices have been dropping more rapidly than prices for tapes and tape drives. In early 2003, hard disks sell for about $1 per gigabyte, which is competitive with prices for blank tapes. This fact makes removable or external hard disks very attractive as a backup medium. Why buy a more expensive and slower tape when you can have a hard disk for the same price? One answer to this question is that removable disks tend to be delicate. A tape is likely to survive a drop from table height, but a hard disk might not. Most removable disk media, such as floppies and Zip disks, are a bit more rugged than hard disks, but they also store less data and are likely to cost more per gigabyte. The delicacy of hard disks can be an important consideration in backup storage, particularly when you take a backup off-site for safekeeping.

You might consider keeping a backup hard disk in the computer to avoid subjecting it to physical shocks. This approach has several drawbacks, though. For one thing, you'll be limited in the number of backups you can retain; one of the beauties of removable backups is that you can keep many backups from different times, which can be handy in reconstructing old work. Another problem is that a hard disk kept inside the computer may be rendered useless by the same problems that might take out the primary disk, such as a power surge or theft of the computer. Nonetheless, an internal backup disk can be part of an overall backup strategy. In fact, some types of Redundant Array of Independent Disks (RAID) configuration are designed as a type of backup with automatic data recovery features.

Another problem with removable hard disks is that they're bulkier and heavier than tapes or other removable disk technologies. This fact can make it more awkward to move the backup to an off-site location. This problem isn't likely to be insurmountable, though; a standard 3.5-inch hard disk is nowhere near as awkward to move as, say, a sofa. You might need to rent a bigger safety deposit box for off-site storage, though, if that's how you handle this task.

Overall, removable hard disks make an appealing backup option, given their falling prices and high speed. If their bulk and delicacy aren't major issues, hard disks are a good choice. For smaller backup jobs, such as archiving individual projects, keeping backups of your configuration files, or transferring individual files between computers, removable disk media such as floppies and Zip disks are good choices. These media are a bit more rugged and compact than hard disks, but they're lower in capacity and slower. Optical media can be a good choice for some of these duties, as well.

0 0

Post a comment