Backup

Backup

A backup schedule is an essential part of any data-protection strategy. You should design a backup system that is right for your situation and the data on your network.

A number of different strategies can be used in backing up files. One way is simply to copy a file to another drive. Operating systems, however, typically have special backup commands that help you with some of the bookkeeping required for maintaining a systematic backup schedule. Most backup commands mark the file with the date and time of the backup so that you (and the backup utility) will know when a copy of the file was saved last. This is the purpose of the FAT file system’s Archive attribute. To determine whether this attribute exists, check the properties of any file on a FAT partition. If the Archive attribute is enabled, the file has changed since the last time a backup was done. In this chapter, you will see that some backup techniques reset this attribute, whereas others do not.

Although backups can be accomplished by saving files to a different drive, they typically are performed with some form of tape drive. Commonly called DAT drives, these devices are able to store many gigabytes of information quickly and economically. Moreover, the tapes are small and portable. Another important step in your backup plan, therefore, is deciding where to store these backup tapes. Many companies choose to make two copies of each backup tape and store one of the copies off-site, thereby guarding against a catastrophic event such as fire.

In addition to the various types of copy commands, Microsoft identifies the following backup types:

Full backup. Backs up all specified files.
Incremental backup. Backs up only those files that have changed since the last backup.
Differential backup. Backs up the specified files if the files have changed since the last backup. This type doesn’t mark the files as having been backed up, however. (A differential backup is somewhat like a copy command. Because the file is not marked as having been backed up, a later differential or incremental backup will back up the file again.)

A typical backup plan includes some combination of these backup types performed at regular intervals. One common practice is to perform an incremental or differential backup each day and a full backup every week. Full backups make the restoration process easier because there is only one set of tapes; however, they also require a lengthy backup process each night, which often means that someone must physically change the tapes.

Incremental backups are much faster because they back up only those files that have been changed since the last backup. The Archive attribute switches on when a file is modified. An incremental backup backs up the file and then removes the attribute so that the file will not be backed up again unless it is changed the next day. A combination of incremental and full backups usually results in four to six incremental tape sets and one full tape set each week. If the drives fail, the administrator must restore the last full backup set, as well as all the incrementals performed since the drive failure. This process obviously is considerably slower than a backup scheme in which a full backup is performed every night.

Differential backups are similar to incrementals except that they do not reset the Archive attribute, which means that each backup during the week backs up all files changed since the last full backup. A full backup once a week (generally Friday or Saturday) and differentials every other day means that only two tapes will be needed in case of failure—the last full backup and the last differential (see Figure 9.1).

Keeping a log of all backups is important. Most backup utilities can generate a backup log. Microsoft recommends that you make two copies of the backup log—store one with the backup tapes and keep one at the computer site. Always test your backup system before you trust it. Perform a sample backup, restore the data, and check the data to be sure it is identical to the original.

You can attach a tape drive directly to a single server, or you can back up several servers across the network at once. Backups over the network are convenient for the administrator, but they can produce considerable network traffic. You can reduce the effects of this extra traffic if you place the computer attached to the tape drive on an isolated network segment and connect it directly to secondary network interface cards on each of the servers.

A number of other vendors also offer backup software—such as Arcada’s BackupExec or Cheyenne’s ArcServe—that include additional features, and in many cases, these are a very wise investment. For the test, though, remember that only the Microsoft Backup utility will be covered.