The first line of defense for fault tolerance is a good tape-backup scheme. Microsoft Windows NT provides a full-function, tape-backup system based on the Maynard tape system. This is a departure from the Sytos OS/2 tape system due to internal limitations imposed by the Sytos system. However, tapes prepared by the Sytos system are readable by Windows NT.
Many tape strategies are available; the correct one for you depends on the volume of data you backup and the frequency of modifications made to the files. There are three types of schemes. Normal backups always copy the selected files and mark each file as having been backed up. This gives you the ability to restore files quickly from the most recent tape. However, this increases the time to make the backups, since even files that have not changed since the last backup are written to the tape. Incremental backups only back up files that have been changed since the last normal or incremental backup. Each file is marked as backed up once copied. This saves time during the backup process. If you combine normal backups with incremental backups, restoring requires starting with your last normal backup and working forward through all the incremental tapes. The last scheme is differential backups. Differential backups back up only those files that have been changed since the last normal or incremental backup. The difference is that the files are not marked as backed up. Combining normal backups with differential backups means that restoration needs only the last normal backup and the last differential.
Rotating sets of tapes used during the backup process is a common practice. It helps reduce the cost of tape backup by reusing tapes over and over again. One of the most popular tape rotation schedules is one that uses 19 tapes over the course of one year. This schedule uses four tapes Monday through Thursday. These are incremental or differential backups. Every Friday a normal backup is created. The tapes used Monday through Thursday are used over and over again. The Friday tapes are kept during the month. On the last Friday of each month, two normal backups are created. One stays in the monthly rotation, the other goes to off-site storage. The tapes in off-site storage stay for one year and rotate back into the cycle to backup the same month they were used for last year. Rotation schemes are as individual as the needs of the organization. Compared to the cost of recovering data, tapes are inexpensive.
With the new tape system there is some new terminology. A backup set is the set of files, directories, or drives selected for a single backup operation. The family set is the set of tapes the backup set resides on. The term is functional for backup sets that require one or more tapes, though usually it is not used unless more than one tape is involved. Information describing the backup set is stored in the catalog. The catalog is always stored on the last or only tape in the family set.
Tape backup and restore are special events with special permissions. Users with read and write permissions may certainly back up and restore certain files. Quite often, however, there is a special user known as the tape operator or backup operator who is called on to interact with the tape. The tape operator (known in Microsoft Windows NT as the Backup Operator) has the privilege to back up and restore files that they do not have read or write permissions to. Users are designated as tape operators by adding their user ID to the Backup Operators group.
Peer services greatly ease the burden of backing up remote computers. If the computer is Microsoft Windows NT or Microsoft Windows for Workgroups, a centralized backup server can backup up remote computers simply by establishing a net use session and backing up the files using the redirected drive letter. The shared directory on the remote computer may be either an entire drive or a subdirectory. If the remote computer does not have sharing capabilities, then the workstation must establish a relationship with either the backup server or some other server. Once the net use has been established, the workstation copies the data to the server. Once on the server, the backup server may either back up the data from a local disk or establish a relationship with the intermediary server and back up the data.
All the tape backup activities are done through the tape backup utility. This is a very simple user interface. Simply click the files you wish to have backed up and select the backup options. The backup dialog screen allows you to specify a tape name, backup set description, and the type of backup. The type corresponds to normal, incremental, or differential options described earlier. The backup can also produce a log of all tape activities.
After selecting which tapes, backup sets, or files to restore, choose the Restore command to open the Restore Information dialog box. The first section provides information about the backup sets. For each backup set you must specify the drive to which you want the information restored. You may also specify an alternate directory path. As with backup, a log is maintained.
Tapes not only make good backup and recovery media, they also make excellent data-transfer facilities. Consider the following problem: Your organization has two sites across town or in adjacent towns. On a daily basis site A has 500MB of data to send to site B. The low-entry cost solution would be to use switched-line modems, say 19,200 bps. It would take (assuming the line stayed up) 7.6 hours a day to transfer the data. A courier with a tape could be across town in an hour in even the worst traffic.
Depending on your organizations, electronic data processing (EDP) auditing requirements or laws and the regulations of your industry, you may be required to keep a set of backups off-site. This allows your organization to pick up the pieces in the event of a catastrophe such as a fire, earthquake, or severe weather. In addition, tapes will most likely play a key roll in re-establishing a disaster recovery site. Here, a site away from a normal operational site is prepared with the equipment needed to run the organization. In the event of a disaster, which cripples the normal operational site, the last tape backups are loaded at the recovery site and the organization continues processing. If disaster recovery procedures are mandated or expected, they should be practiced regularly to ensure the current staff is well-trained.