Backup Types and Devices

Most “live” data in use today are stored on hard disk drives. While the reliability of the hard disk devices found in desktop and laptop systems has steadily improved over the last decade, they are nevertheless mechanical devices quite capable of wearing out, sometimes prematurely, sometimes without warning. Furthermore, users are only human, often lacking in formal training. Sometimes they erase important files or records within files by mistake. Sometimes they delete data out of malice. Viruses and other malicious programs can destroy files. Making backup copies of all of the files that are on a hard disk is the best, and often the only, means of recovery from mechanical failure, user error, malevolent software, natural disaster, and physical theft.

Hard drives have finite storage capacity. Eventually you have to erase files from the hard disk to make way for more. You may need to keep copies of those “surplus” files, such as last year’s bookkeeping ledger. These days some people use two computers, one on the desk at work, another that travels with the user or resides in the user’s home. Thus we can identify at least four different types of file copying, as listed in Exhibit 1.

The main focus in this section is backups, but the other categories are also important. Updates that synchronize files between desktops and portable machines are a relatively recent concern and have implications for data integrity. An archive is a set of files that has been copied as an historical record. Typically these are files containing data that will not change, and immediate access to which is no longer required, such as properly aged accounting records. When the archive copy has been created the original can be erased, thus freeing up storage space. Several terms that are useful at this point are

•  Primary storage — where frequently used software and data reside.

•  Online storage — storage that is immediately available and randomly accessible, this includes removable media such as floppy diskettes.

•  Removable media — any media that can be physically removed from the system, such as diskettes and CD-ROMs.

•  Magnetic media — storage based on magnetic properties, such as hard drives, tapes, and floppies.

•  Optical media — storage based on optical properties, such as CD-ROMs.

•  Magneto-optical — storage based on a combination of magnetic and optical properties, like some high-capacity cartridge drives.

•  Random vs. linear access — the ability to immediately access data regardless of their physical location on the media (e.g., a hard drive) as opposed to access which requires reading preceding data (e.g., a tape drive).

•  Read only — the ability to read stored data but not change it.

•  Write once, read many — the ability to record data in read only form and then read it multiple times (e.g., burning a CD-ROM).

•  RAID — redundant array of inexpensive disks — a storage system which combines multiple disks managed as a single storage device, allowing disks to be “hot swapped,” i.e., replaced without powering down or losing data.

•  Jukebox — a storage system which combines multiple tapes or CD-ROM drives managed as a single storage device with automated media switching, providing large-scale storage or backup.

In the early days of personal computing the primary means of backup, software duplication, and archiving, was the floppy diskette. A floppy diskette can be described as randomly accessible removable media, with write many/read many, as well as read only capability (by physically adjusting the write-protect setting on the disk jacket you can write-protect the contents, although this is a reversible procedure, distinguishable from WORM media that is physically impossible to overwrite). The floppy diskette has several benefits:

•  Low cost for both drives and media

•  Included as standard equipment on all machines

•  Widespread compatibility between systems

Unfortunately, hard drive capacities and the complexity of both software and data have far outstripped the capacity of standard diskettes, while possible alternatives such as high-capacity cartridge drives and read/write optical media have so far failed to achieve anything like the same level of acceptance as standard equipment. The current options for backup are listed in Exhibit 2. Note that some of these removable media devices also work as primary storage, for active software and live data, as well as secondary or backup storage.

While constant improvements in performance, capacity, and pricing make “best buy” statements about storage devices imprudent, there are clearly some practical points that can be made. First of all, you need to match capacity and speed to need. For example, if a desktop machine uses about 600 megabytes of hard drive storage, 5 megabytes of which is updated every day, a CD-R drive might be worth considering as an alternative to tape. But tape would be better for a system that regularly stores twice as much data and updates data at a faster daily rate. For a network file server that stores several gigabytes of constantly changing data, you will probably want to use RAID for primary storage and a jukebox for constant backup.¹³

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¹³A tape jukebox can cycle through multiple tapes and backup RAID data that is mirrored and not being accessed.

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