No single backup policy can meet the needs of all users. A policy that works well for a system with one user, for example, could be inadequate for a system that serves 5 or 10 different users. Likewise, a policy developed for a system on which many files are changed daily would be inefficient for a system on which data changes infrequently. Only you can determine the best backup policy for your system, but the following general guidelines should help:
Make sure you can recover from major losses.
Can your system continue to run after any single fixed disk fails? Can you recover your system if all the fixed disks should fail? Could you recover your system if you lost your backup diskettes or tape to fire or theft? Although these things are not likely, any of them are possible. Think through each of these possible losses and design a backup policy that would enable you to recover your system after any of them.
Check your backups periodically.
Backup media and its hardware can be unreliable. A large library of backup tapes or diskettes is useless if their data cannot be read back onto a fixed disk. To make certain that your backups are usable, try to display the table of contents from the backup tape periodically (using restore -T, or tar -t for archive tapes). If you use diskettes for your backups and have more than one diskette drive, try to read diskettes from a different drive than the one on which they were created. You also may want the security of repeating each level 0 backup with a second set of diskettes. If you use a streaming tape device for backups, you can use the tapechk command to perform rudimentary consistency checks on the tape.
Keep old backups.
Develop a regular cycle for reusing your backup media; however, you should not reuse all of your backup media. Sometimes it may be months before you or some other user of your system notices that an important file is damaged or missing. You should save old backups for such possibilities. For example, you could have the following three cycles of backup tapes or diskettes:
Check file systems before backing them up.
A backup that was made from a damaged file system may be useless. Before making your backups, it is good policy to check the integrity of the file system with the fsck command.
Ensure files are not in use during a backup.
Your system should not be in use when you make your backups. If the system is in use, files can change while they are being backed up, and the backup copy will not be accurate.
Back up your system before major changes are made to the system.
It is always good policy to back up your entire system before any hardware testing or repair work is performed or before you install any new devices, programs, or other system features.
Other Factors
Other items to consider when planning and implementing a backup strategy are:
Whatever the appropriate backup strategy for your site, it is very important that one exists. Backups should be done frequently and regularly. Recovering from data loss is very difficult if a good backup strategy has not been implemented.
Several different types of backup media are available for backups. The different types of backup media available to your specific system configuration depend upon both your software and hardware. The types most frequently used are the 5.25-inch diskette, 8-mm tape, 9-track tape, and the 3.5-inch diskette.
Attention: Running the backup command results in the loss of all material previously stored on the selected output medium.
The following shows different types of backup media.
Diskettes are the standard backup medium. Unless you specify a different device using the backup -f command, the backup command automatically writes its output to the /dev/rfd0 device, which is the diskette drive. To back up to the default tape device, enter /dev/rmt0.
Be careful with your diskettes. Because each piece of information occupies such a small area on the diskette, small scratches, dust, food, or tobacco particles can make the information unusable. Be sure to remember the following:
Attention: Diskette drives and diskettes must be the correct type to store data successfully. If you use the wrong diskette in your 3.5-inch diskette drive, the data on the diskette could be destroyed.
The diskette drive uses the following 3.5-inch diskettes:
Tape is well-suited to certain tasks because of its high capacity and durability. It is often chosen for storing large files or many files, such as archive copies of file systems. It is also used for transferring many files from one system to another. Tape is not widely used for storing frequently accessed files because other media provide much faster access times.
Tape files are created using commands such as backup, cpio, and tar, which open a tape drive, write to it, and close it.