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AIX Version 4.3 System Management Guide: Operating System and Devices

Backup Overview

Once your system is in use, your next consideration should be backing up file systems, directories, and files. Files and directories represent a significant investment of time and effort. At the same time, all computer files are potentially easy to change or erase, either intentionally or by accident. If you take a careful and methodical approach to backing up your file systems, you should always be able to restore recent versions of files or file systems with little difficulty. When a hard disk crashes, the information contained on that disk is destroyed. The only way to recover the destroyed data is to retrieve the information from your backup copy.

The following backup and restore procedures are also provided:

Developing a Backup Strategy

Compressing Files

Backing Up User Files or File Systems

Backing Up the System Image and User-Defined Volume Groups

Backing Up a User Volume Group

Implementing Scheduled Backups

Restoring from Backup Image Individual User Files

Backup Methods

Several different methods exist for backing up information. One of the most frequently used methods is called backup by name, also called file name archive. This method of backup is done when the i flag is specified and is used to make a backup copy of individual files and directories. It is a method commonly used by individual users to back up their accounts.

Another frequently used method is called backup by file system, also called backup by i-node or file system archive. This method of backup is done when the i flag is not specified. It is used to make a backup copy of an entire file system and is the method commonly used by system administrators to back up large groups of files, such as all of the user accounts in /home. A file system backup allows incremental backups to be performed easily. An incremental backup will back up all files that have been modified since a specified previous backup.

The compress and pack commands enable you to compress files for storage, and the uncompress and unpack commands unpack the files once they have been restored. The process of packing and unpacking files takes time, but once packed the data uses less space on the backup medium.

Several commands create backups and archives. Because of this, data that has been backed up needs to be labeled as to what command was used when doing the backup, and how the backup was made (by name or by file system). The backup command is the most frequently used, but other commands serve specific purposes:

backup Backs up files by name or by file system.
mksysb Creates an installable image of the rootvg volume group.
cpio Copies files into and out of archive storage. Can usually read data archived on another platform provided it is in cpio format.
dd Converts and copies a file. Commonly used to convert and copy data to and from non-AIX systems, for example, mainframes. dd does not group multiple files into one archive; it is used to manipulate and move data.
tar Manipulates tar format archives.
rdump A network command that backs up files by file system onto a remote machine's device.
pax POSIX-conformant archive utility that can read and write tar and cpio archives.

Deciding on a Backup Policy

No single backup policy can meet the needs of all users. A policy that works well for a system with one user, for example, could be inadequate for a system that serves five or ten different users. Likewise, a policy developed for a system on which many files are changed daily would be inefficient for a system on which data changes infrequently. Whatever the appropriate backup strategy for your site, it is very important that one exist and that backups be done frequently and regularly. It is difficult to recover from data loss if a good backup strategy has not been implemented.

Only you can determine the best backup policy for your system, but the following general guidelines should help:

Attention: System backup and restore procedures require that the system be restored on the same type of platform from which the backup was made. In particular, the CPU and I/O planar boards must be of the same type. The backup and restore procedures described herein were tested for operation on the IBM RS/6000 platform and may not operate as described on other platforms.

Understanding Backup Media

Several types of backup media are available. The types of backup media available to your specific system configuration depend upon your software and hardware. The types most frequently used are 8-mm tape, 9-track tape, and the 3.5-inch diskette.

For backing up individual files and file systems, diskettes are the standard medium. Unless you specify a different device using the backup -f command, the backup command automatically writes its output to /dev/rfd0, which is the diskette drive. To back up to the default tape device, use /dev/rmt0.

Attention: Running the backup command results in the loss of all material previously stored on the selected output medium.

Restoring Data

Once data has been properly backed up, there are several different methods of restoring the data based upon the type of backup command you used.

You need to know how your backup or archive was created to restore it properly. Each backup procedure gives information about restoring data. For example, if you use the backup command, you can specify a backup either by file system or by name. That backup must be restored the way it was done, by file system or by name.

Several commands restore backed up data, such as:

restore Copies files created by the backup command.
rrestore Network command that copies file systems backed up on a remote machine to the local machine.
cpio Copies files into and out of archive storage.
tar Manipulates archives. Used only for directories.

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