Topics covered in this tutorial

• UPS and power protection
• Backup and backup procedures
• Disk fault tolerance, including RAID
• Virus protection

Fault Tolerance and Redundancy

Network systems also require protection from system failure and external problems. Examples of devices and techniques that fall into this category include the following:

• Uninterruptible power supplies (UPS)
• Drive arrays - RAID
• Backup strategies
• Virus prevention

Backup Strategies



One of the most important operations in a network system is the creation of a secure backup. Typically, backups take place using a tape system that has the advantage of high capacity, relatively low cost and portability.

Backup Methods



A backup may be performed using one of three methods:

• Full
• Incremental
• Differential

A full backup includes all selected files and directories while incremental and differential backups check the status of the archive attribute before including a file.

The archive attribute is set whenever a file is modified. This allows backup software to determine which files have been changed, and therefore need to be copied.

The criteria for determining which method to use is based on the time it takes to restore versus the time it takes to back up.

Assuming a backup is performed every working day, an incremental backup only includes files changed during that day, while a differential backup includes all files changed since the last full backup.

Incremental backups save backup time but can be more time-consuming when the system must be restored. The system must be restored from the last full backup set and then from each incremental backup that has subsequently occurred. A differential backup system only involves two tape sets when restore is required.

The following table summarizes the three different backup types:



Doing a full everyday backup on a large network takes a long time. A typical strategy for a complex network would be a full weekly backup followed by an incremental or differential backup at the end of each day.

• The advantage of using a full daily backup is that one tape set is only required to restore the system
• The advantage of an incremental backup is that it takes less time to back up but several tape sets may need to be restored before the system is operational
• The advantage of a differential backup is the balance of time for both restoring and backing up

Do not combine differential and incremental backups. Use full backups interspersed with differential backups, or full backups interspersed with incremental backups.