19.10. Backup Basics

The major backup programs built into FreeBSD are dump(8), tar(1), cpio(1), and pax(1).

19.10.1. Dump and Restore

The traditional UNIX® backup programs are dump and restore. They operate on the drive as a collection of disk blocks, below the abstractions of files, links and directories that are created by the file systems. Unlike other backup software, dump backs up an entire file system on a device. It is unable to backup only part of a file system or a directory tree that spans more than one file system. dump does not write files and directories, but rather writes the raw data blocks that comprise files and directories. When used to extract data, restore stores temporary files in /tmp/ by default. When using a recovery disk with a small /tmp, set TMPDIR to a directory with more free space in order for the restore to succeed.

Σημείωση:

If dump is used on the root directory, it will not back up /home, /usr or many other directories since these are typically mount points for other file systems or symbolic links into those file systems.

dump has quirks that remain from its early days in Version 6 of AT&T UNIX®,circa 1975. The default parameters are suitable for 9-track tapes (6250 bpi), not the high-density media available today (up to 62,182 ftpi). These defaults must be overridden on the command line to utilize the capacity of current tape drives.

It is also possible to backup data across the network to a tape drive attached to another computer with rdump and rrestore. Both programs rely upon rcmd(3) and ruserok(3) to access the remote tape drive. Therefore, the user performing the backup must be listed in .rhosts on the remote computer. The arguments to rdump and rrestore must be suitable to use on the remote computer. For example, to rdump from a FreeBSD computer to an Exabyte tape drive connected to a host called komodo, use:

# /sbin/rdump 0dsbfu 54000 13000 126 komodo:/dev/nsa8 /dev/da0a 2>&1

There are security implications to allowing .rhosts authentication, so use with caution.

It is also possible to use dump and restore in a more secure fashion over ssh.

Παράδειγμα 19.1. Using dump over ssh
# /sbin/dump -0uan -f - /usr | gzip -2 | ssh -c blowfish \
          targetuser@targetmachine.example.com dd of=/mybigfiles/dump-usr-l0.gz

Or, use the built-in RSH:

Παράδειγμα 19.2. Using dump over ssh with RSH Set
# env RSH=/usr/bin/ssh /sbin/dump -0uan -f targetuser@targetmachine.example.com:/dev/sa0 /usr

19.10.2. tar

tar(1) also dates back to Version 6 of AT&T UNIX®, circa 1975. tar operates in cooperation with the file system and writes files and directories to tape. tar does not support the full range of options that are available from cpio(1), but it does not require the unusual command pipeline that cpio uses.

To tar to an Exabyte tape drive connected to a host called komodo:

# tar cf - . | rsh komodo dd of=tape-device obs=20b

When backing up over an insecure network, instead use ssh.

19.10.3. cpio

cpio(1) is the original UNIX® file interchange tape program for magnetic media. cpio includes options to perform byte-swapping, write a number of different archive formats, and pipe the data to other programs. This last feature makes cpio an excellent choice for installation media. cpio does not know how to walk the directory tree and a list of files must be provided through stdin.

Since cpio does not support backups across the network, use a pipeline and ssh to send the data to a remote tape drive.

# for f in directory_list; do
find $f >> backup.list
done
# cpio -v -o --format=newc < backup.list | ssh user@host "cat > backup_device"

Where directory_list is the list of directories to back up, user@host is the user/hostname combination that will be performing the backups, and backup_device is where the backups should be written to, such as /dev/nsa0).

19.10.4. pax

pax(1) is the IEEE/POSIX® answer to tar and cpio. Over the years the various versions of tar and cpio have become slightly incompatible. So rather than fight it out to fully standardize them, POSIX® created a new archive utility. pax attempts to read and write many of the various cpio and tar formats, plus new formats of its own. Its command set more resembles cpio than tar.

19.10.5. Amanda

Amanda (Advanced Maryland Network Disk Archiver) is a client/server backup system, rather than a single program. An Amanda server will backup to a single tape drive any number of computers that have Amanda clients and a network connection to the Amanda server. A common problem at sites with a number of large disks is that the length of time required to backup to data directly to tape exceeds the amount of time available for the task. Amanda solves this problem by using a «holding disk» to backup several file systems at the same time. Amanda creates «archive sets»: a group of tapes used over a period of time to create full backups of all the file systems listed in Amanda's configuration file. The «archive set» also contains nightly incremental, or differential, backups of all the file systems. Restoring a damaged file system requires the most recent full backup and the incremental backups.

The configuration file provides fine grained control of backups and the network traffic that Amanda generates. Amanda will use any of the above backup programs to write the data to tape. Amanda is not installed by but is available as either a port or package.

19.10.6. Do Nothing

«Do nothing» is not a computer program, but it is the most widely used backup strategy. There are no initial costs. There is no backup schedule to follow. Just say no. If something happens to your data, grin and bear it!

If your time and data is worth little to nothing, then «Do nothing» is the most suitable backup program for the computer. But beware, FreeBSD is a useful tool and over time it can be used to create a valuable collection of files.

«Do nothing» is the correct backup method for /usr/obj and other directory trees that can be exactly recreated by the computer. An example is the files that comprise the HTML or PostScript® version of this Handbook. These document formats have been created from XML input files. Creating backups of the HTML or PostScript® files is not necessary if the XML files are backed up regularly.

19.10.7. Which Backup Program Is Best?

dump(8) Period. Elizabeth D. Zwicky torture tested all the backup programs discussed here. The clear choice for preserving all your data and all the peculiarities of UNIX® file systems is dump. Elizabeth created file systems containing a large variety of unusual conditions (and some not so unusual ones) and tested each program by doing a backup and restore of those file systems. The peculiarities included: files with holes, files with holes and a block of nulls, files with funny characters in their names, unreadable and unwritable files, devices, files that change size during the backup, files that are created/deleted during the backup and more. She presented the results at LISA V in Oct. 1991. See torture-testing Backup and Archive Programs.

19.10.8. Emergency Restore Procedure

19.10.8.1. Before the Disaster

There are four steps which should be performed in preparation for any disaster that may occur.

First, print the bsdlabel of each disk using a command such as bsdlabel da0 | lpr. Also print a copy of /etc/fstab and all boot messages.

Second, burn a «livefs» CD. This CD contains support for booting into a FreeBSD «livefs» rescue mode, allowing the user to perform many tasks like running dump(8), restore(8), fdisk(8), bsdlabel(8), newfs(8), mount(8), and more. The livefs CD image for FreeBSD/i386 9.2-RELEASE is available from ftp://ftp.FreeBSD.org/pub/FreeBSD/releases/i386/ISO-IMAGES/9.2/FreeBSD-9.2-RELEASE-i386-livefs.iso.

Σημείωση:

Livefs CD images are not available for FreeBSD 10.0-RELEASE and later. In addition to the CD-ROM installation images, flash drive installation images may be used to recover a system. The «memstick» image for FreeBSD/i386 10.0-RELEASE is available from ftp://ftp.FreeBSD.org/pub/FreeBSD/releases/i386/i386/ISO-IMAGES/10.0/FreeBSD-10.0-RELEASE-i386-memstick.img.

Third, create backup tapes regularly. Any changes that made after the last backup may be irretrievably lost. Write-protect the backup media.

Fourth, test the «livefs» CD and the backups. Make notes of the procedure. Store these notes with the CD, the printouts, and the backups. These notes may prevent the inadvertent destruction of the backups while under the stress of performing an emergency recovery.

For an added measure of security, store an extra «livefs» CD and the latest backup at a remote location, where a remote location is not the basement of the same building. A remote location should be physically separated from the computers and disk drives by a significant distance.

19.10.8.2. After the Disaster

First, determine if the hardware survived. Thanks to regular, off-site backups, there is no need to worry about the software.

If the hardware has been damaged, the parts should be replaced before attempting to use the computer.

If the hardware is okay, insert the «livefs» CD and boot the computer. The original install menu will be displayed on the screen. Select the correct country, then choose Fixit -- Repair mode with CD-ROM/DVD/floppy or start a shell. then select CD-ROM/DVD -- Use the live filesystem CD-ROM/DVD. restore and the other needed programs are located in /mnt2/rescue.

Recover each file system separately.

Try to mount the root partition of the first disk using mount /dev/da0a /mnt. If the bsdlabel was damaged, use bsdlabel to re-partition and label the disk to match the label that was printed and saved. Use newfs to re-create the file systems. Re-mount the root partition of the disk read-write using mount -u -o rw /mnt. Use the backups to recover the data for this file system. Unmount the file system with umount /mnt. Repeat for each file system that was damaged.

Once the system is running, backup the data onto new media as whatever caused the crash or data loss may strike again. Another hour spent now may save further distress later.

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