23.1 Types of Backup | HP Certified: HP-UX System Administration

While creating a backup, you may not need to create a full system backup every day. This is because much of the data on your system are static in nature and do not change often. Examples of static data are system binary files and applications installed on your system. However, it is better to keep one full backup of your system just after the installation. You may also want to prepare a full backup every month. However, on a weekly basis, you can back up only those files that have been changed since the last monthly backup. Similarly, daily backups may contain only those files that have been changed since the last weekly backup.

The method of taking a partial backup is called an incremental backup. In an incremental backup scheme, different backup levels are defined. Backup level 0 is a full backup. If you perform a backup at level 1 after taking a full backup at backup level 0, only those files are backed up that were changed or created after taking the full backup. Similarly, if you perform a backup at level 2 after you have performed a backup at level 1, those files are backed up that have changed since backup level 1. However, keep in mind that all these backup levels are relative to each other. For example, if you have never performed a backup at level 0 or 1, a backup taken at backup level 2 will be a full backup. Every backup that is not a full backup is an incremental backup. A complete backup plan is usually a combination of a full backup and one or more incremental backups.

Full Backup

When you perform a backup using a backup level, its information is stored in a file that keeps a record of the date and time along with the backup level. This information is used when the next backup is taken to check whether a backup at a higher level has already been taken. For example, if you are using the fbackup command to back up a system, the information is stored in the /var/adm/fbackupfiles/dates file. A full backup is always taken if there is no record of any backup at a higher backup level. However, since the topmost backup level is 0, a backup taken at level 0 is always a full backup. It is, therefore, a general convention that a full backup is always taken at backup level 0.

A full backup can take a considerably long time depending on how much disk space is used with your system. You should design a backup schedule such that the full backup is performed when there are few or no users logged into the system. It is better to bring the system to single- user mode before starting a backup. If you are taking a full backup every month, the first weekend of the month may be a good time to prepare a full backup.

Incremental Backup

An incremental backup is a partial system backup where only those files are backed up that have been modified or created since the last backup at a higher level. If you are using level 0 for a full backup, you can use backup level 1 for a weekly backup to be carried out on a Friday night. For a daily backup, you may use backup level 2, which will back up only those files that have been changed since last Friday.

Incremental backup is useful for weekly and daily routines. Incremental backup takes much less time as only a part of the system is backed up. It also consumes less backup media. For example, you may need only one tape cartridge for an incremental backup, whereas a number of tape cartridges may be required for a full backup depending on how much disk capacity you are using with your system.

Recovering from an Incremental Backup

The method of recovery using an incremental backup involves more than one step, depending on how many backup levels you are using for your system. Let's suppose that you are using three backup levels as shown here.

Backup level 0	Used for taking a full backup on the first weekend of every month.
Backup level 1	Used for a weekly backup on Friday night.
Backup level 2	Used for a daily backup every night at 11 p.m.

If your system goes down on Thursday, you will perform three steps to fully restore the data. These three steps are:

Restore the monthly backup first. This will bring your system to the state it was at the first weekend of the month.
Restore the weekly backup taken on the previous Friday night. This will bring your system back to the state it was in last Friday.
Restore the daily backup taken yesterday (Wednesday), and it will bring your system back to the state at which it was at the end of yesterday . The only data you lose are the data entered today.

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