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System Management Concepts:
Operating System and Devices

Backup Overview

Backing up file systems, directories, and files represents 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. 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. If you carefully and methodically back up your file systems, you can always restore recent versions of files or file systems with little difficulty.

Backup Methods

Several methods exist for backing up information. One of the most frequently used methods is called backup by name or 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 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 backs 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 which command was used to initiate the backup, and how the backup was made (by name or by file system).

backup Backs up files by name or by file system.
mksysb Creates an installable image of the rootvg.
cpio Copies files into and out of archive storage.
dd Converts and copies a file. Commonly used to convert and copy data to and from systems running other operating systems, for example, mainframes. dd does not group multiple files into one archive; it is used to manipulate and move data.
tar Creates or manipulates tar format archives.
rdump Backs up files by file system onto a remote machine's device.
pax (POSIX-conformant archive utility) Reads and writes 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 one hundred 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 might be helpful:

For the backup of named pipes (FIFO special files) the pipes can be either closed or open. However, the restoration fails when the backup is done on open named pipes. When restoring a FIFO special file, its i-node is all that is required to recreate it because it contains all its characteristic information. The content of the named pipe is not relevant for restored. Therefore, the file size during backup is zero (all the FIFOs closed) before the backup is made.
Attention: System backup and restoration 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.

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 tapes, diskettes, remote archives, and alternate local hard disks. 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.

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

Restoring Data

After the data has been correctly 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 correctly. 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 Copies file systems backed up on a remote machine to the local machine.
cpio Copies files into and out of archive storage.
tar Creates or manipulates tar archives.
pax (POSIX-conformant archive utility) Reads and writes tar and cpio archives.

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