September 1993

Foolproof Data Backup

Nobody likes to lose computer data. That's why a foolproof system for backing up your data -- that is, making a copy of it -- seems like a good idea. Unfortunately, no system is foolproof. A recent magazine story made this point very well.

The article described a network file server which had a tape drive for regular backup of its hard disk. Each night, the tape drive did a complete backup of the file server's disk, and each morning a new tape was placed in the tape drive. This is a good procedure for file servers.

The file server also had mirrored disk drives. Mirrored drives, sometimes called "RAID Level One", are two disk drives which both store exactly the same information. If one drive fails, the other one still has your data intact. The computer keeps running while you wait for the service people to replace the failed drive. Because of the added cost, many servers do not have such redundancy.

So far this seems like a good system, but recall Murphy's Law: Whatever can go wrong, will.

Let's call the mirrored disks A and B. First disk B failed, but the person in charge of the system did not know it. Months later disk A also failed and the file server stopped working. When service personnel arrived on the scene, they assumed only one disk was bad. They determined that disk A was not working. To test disk A, they wrote over the data, because they thought disk B had the same data. This left the system with no data on disk A and months-old data on a defective disk B.

As bad as this sounds, the tape backup should have come to the rescue. The previous night's tape should have had all the data, so at worst one day's data would have been lost. But the tape drive had not been making regular backups. No one had checked the backup tapes to be sure they were being written and could be read.

Is Your Data Safe?

Suppose your computer system stopped working right now. What would you lose? An hour's work? A day's work? A week's work? Could you take a disk or tape with your data to another computer and use it there?

Or suppose your office were completely destroyed by fire. Do you have backup tapes stored off-site? Could you buy a compatible tape drive to read your backup tapes into a new computer? Do you even know what kind of tape drive you have, and what software it uses?

For most systems a reasonable response might be that you could recover all your data as of the previous night from a backup tape, should your computer totally fail and be inoperable for days. In case of disaster, you should lose no more than one week of data by relying on weekly off-site storage of a backup tape. That assumes you can read the tape on a new computer.

Local Hard Disk Backup

Thus far we have discussed file server backup. However, many people have valuable data on the "local" hard disk of their PC, whether or not that PC is attached to a network. How can you safeguard it?

Networks normally do not back up each user's hard disk. To do so requires extra software on each PC, taking away memory space, which is usually in short supply on networked DOS computers. Also, most users do not leave their PC on at night, when a backup would be done. Finally, it can take a lot of time and a high-capacity tape drive to back up all the PCs on a large network.

If you are on a network, perhaps you can copy your data to the file server. Your system administrator will have to provide a place for your data and a time for you to do the backup.

If you are not connected to a network, or if you have a large quantity of data on a networked PC, a tape drive is essential. Tape drive prices have dropped a lot. A 250-megabyte tape drive from Colorado Memory Systems can be purchased for under $200. Back up the entire hard disk once a week.

Daily Floppy Backup

Most people can use floppy disks for daily backup of files which have changed. A 3.5" DOS disk holds 1.44 megabytes, about 700 typewritten pages.

The following DOS command will copy, to the floppy disk in drive B, all files in directory C:\DOCS which have changed since the last backup:


If you do a weekly tape backup (and set the software properly), the XCOPY command will only copy files which changed after the tape backup. If you need to exclude other files (such as large temporary files written by some programs), you can use the DOS command ATTRIB.

A typical computer user can do daily backups to the same floppy disk for an entire week. Each backup will only copy changed files, so it should be fast. If you change a file repeatedly, the floppy disk will have the latest version. However, database files are sometimes too large for one floppy disk. Then backup to a file server or tape is preferable.

Automated Backup

As much as possible, backup should be automated, because people find it boring and therefore neglect to do it. Backups to a network, to tape, or to floppy disk can all be automated with appropriate "batch" files and software which runs at a pre-determined time.

Call RTG if you would like us to help you set up an automated procedure to handle your backup under DOS, networks, or UNIX.

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