21.4. Determining a Backup Tape Rotation Scheme

Backups must be performed on a disciplined, scheduled regimen, carefully designed to address certain issues, including the following:

• The schedule for backups

• Whether to use partial or full backups

• Whether partial backups should be incremental or differential

The terms full, partial, incremental, and differential describe the amount of information that is copied and backed up. All backups, whether full or partial, can be performed online or offline.

The archive bit is a file attribute that identifies whether the file has been backed up. If a file is changed after the last backup, the archive bit is cleared until the file is backed up again.

Full backups back up all the selected data on the hard drive and reset the archive bit.

Differential backups back up all the data since the last full backup but do not reset the archive bit.

Incremental backups back up only the changed data from the last full backup and reset the archive bit.

21.4.1 Full Backup

A full backup is a complete backup of the entire server or client. A server backup includes all volumes, directories, and files. A client backup includes all drives, directories, and files.

The two types of full backups are normal backup and copy backup.

21.4.1.1 NORMAL BACKUP

A normal backup backs up database files and then the transaction log files. It then deletes the transaction log files from the directory. You can have circular logging disabled because your backup software deletes the log files. Therefore, if you are performing regular backups, you will not have a problem with log files filling your drive. To restore a normal backup, you only need to restore your last normal backup set and then start the service.

21.4.1.2 COPY BACKUP

A copy backup is similar to a normal backup, but it does not purge the log files on your drive and does not update the backup context in the database files. A copy backup is practical when you want to back up your data without disrupting your normal backup schedule.

21.4.2 Partial Backup

A partial backup can be either incremental or differential.

21.4.2.1 INCREMENTAL

An incremental backup copies all files that were changed after the last backup, regardless of what kind of backup it was.

Use an incremental backup when you must maintain every revision of a file. If you use the same tapes for consecutive incremental backups, do not allow the newer versions of backed-up files to overwrite earlier versions. Instead, append the newer files to the backup medium.

An incremental backup works only on the log files. Like a normal backup, an incremental backup purges log files after backing them up, providing another way to rid log files from your drive without compromising recoverability.

To restore an incremental backup, you must return to your last normal backup set that contains your database files. Restore those database files, restore every incremental backup set made after the normal backup, and then start the service. Do not start the service until you have restored all the backup sets; otherwise any logs restored after the backup set will not be played forward.

21.4.2.2 DIFFERENTIAL

A differential backup copies all files that were changed after the last complete backup.

Use differential backups when you only need to save the latest version of each file. If you use the same tapes for consecutive differential backups, the newer versions of backed-up files often overwrite older versions of the same file on the tape. Backup programs typically do not reset the archive bit after a differential backup. Rather, the archive bit remains enabled until the next complete backup.

To restore a differential backup, return to the last normal backup and restore the differential backup that contains log files generated after the last normal backup. Do not start the service until you have restored all the backup sets.

21.4.3 Grandfather-Father-Son Tape Rotation

The grandfather-father-son (GFS) tape rotation scheme is the most commonly used scheme, requiring a weekly backup capacity of at least double the server storage capacity. Providing different levels of data retention, this scheme uses three levels of backup to supply redundancy and security.

The system administrator can select which generation of tapes to store temporarily and which to archive.

GFS backup requires the following:

1. Monthly grandfathers

2. Weekly fathers

3. Daily sons

Example

The system administrator typically performs a full backup every Monday (father) and incremental backups on Tuesdays, Wednesdays, and Thursdays (sons). The administrator performs another full backup at the end of the week (father) and another at the end of the month (grandfather).

Note

The GFS tape rotation scheme is intended to ensure that a company can always restore lost data to within a day of a disaster. Businesses dedicated to helping get a company back online after a disaster can make computer equipment, tape drives, phone equipment, and so forth available to a company quickly.

The tapes containing the weekly and monthly backups are usually stored in a location away from the site of the server. To help reduce media costs, many companies reuse older weekly backup tapes.

Warning

Never overwrite a recently used backup tape. If a hard disk crash should occur during the backup, not only would all the data on the disk be lost, but also the tape will no longer be useful for a complete restoration.

In some cases, customer requirements might be better met by performing differential backups for the daily sons rather than incremental backups. In a few cases, daily full backups might be required. The trade-offs between the various types of partial and complete backups relate to the following:

• Amount of data that will be backed up each day

• Time required to complete the backup session

• Number of tapes required to recover data to the server after a disaster