12.9 Backup Basics

12.9.1 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. 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 to tape, but rather writes the raw data blocks that comprise files and directories.

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 and ruserok to access the remote tape drive. Therefore, the user performing the backup must be listed in the .rhosts file on the remote computer. The arguments to rdump and rrestore must be suitable to use on the remote computer. When rdumping from a FreeBSD computer to an Exabyte tape drive connected to a Sun called komodo, use:

    # /sbin/rdump 0dsbfu 54000 13000 126 komodo:/dev/nrsa8 /dev/rda0a 2>&1

Beware: there are security implications to allowing .rhosts authentication. Evaluate your situation carefully.

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

    # /sbin/dump -0uan -f - /usr | gzip -2 | ssh1 -c blowfish \
          [email protected] dd of=/mybigfiles/dump-usr-l0.gz

12.9.2 tar

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

Most versions of tar do not support backups across the network. The GNU version of tar, which FreeBSD utilizes, supports remote devices using the same syntax as rdump. To tar to an Exabyte tape drive connected to a Sun called komodo, use:

    # /usr/bin/tar cf komodo:/dev/nrsa8 . 2>&1

For versions without remote device support, you can use a pipeline and rsh to send the data to a remote tape drive.

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

If you are worried about the security of backing up over a network you should use the ssh command instead of rsh.

12.9.3 cpio

cpio(1) is the original Unix file interchange tape program for magnetic media. cpio has options (among many others) to perform byte-swapping, write a number of different archive formats, and pipe the data to other programs. This last feature makes cpio and 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.

cpio does not support backups across the network. You can use a pipeline and rsh 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 you want 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 (e.g., /dev/nrsa0).

12.9.4 pax

pax(1) is IEEE/POSIX's answer to tar and cpio. Over the years the various versions of tar and cpio have gotten 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.

12.9.5 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. Amanda can use 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 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 available as either a port or a package, it is not installed by default.

12.9.6 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 your data is worth little to nothing, then ``Do nothing'' is the most suitable backup program for your computer. But beware, Unix is a useful tool, you may find that within six months you have a collection of files that are valuable to you.

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

12.9.7 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 filesystems 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.

12.9.8 Emergency Restore Procedure

    #!/bin/sh
    #
    # create a restore floppy
    #
    # format the floppy
    #
    PATH=/bin:/sbin:/usr/sbin:/usr/bin
    
    fdformat -q fd0
    if [ $? -ne 0 ]
    then
         echo "Bad floppy, please use a new one"
         exit 1
    fi
    
    # place boot blocks on the floppy
    #
    disklabel -w -B /dev/fd0c fd1440
    
    #
    # newfs the one and only partition
    #
    newfs -t 2 -u 18 -l 1 -c 40 -i 5120 -m 5 -o space /dev/fd0a
    
    #
    # mount the new floppy
    #
    mount /dev/fd0a /mnt
    
    #
    # create required directories
    #
    mkdir /mnt/dev
    mkdir /mnt/bin
    mkdir /mnt/sbin
    mkdir /mnt/etc
    mkdir /mnt/root
    mkdir /mnt/mnt          # for the root partition
    mkdir /mnt/tmp
    mkdir /mnt/var
    
    #
    # populate the directories
    #
    if [ ! -x /sys/compile/MINI/kernel ]
    then
         cat << EOM
    The MINI kernel does not exist, please create one.
    Here is an example config file:
    #
    # MINI -- A kernel to get FreeBSD onto a disk.
    #
    machine         "i386"
    cpu             "I486_CPU"
    ident           MINI
    maxusers        5
    
    options         INET                    # needed for _tcp _icmpstat _ipstat
                                            #            _udpstat _tcpstat _udb
    options         FFS                     #Berkeley Fast File System
    options         FAT_CURSOR              #block cursor in syscons or pccons
    options         SCSI_DELAY=15           #Be pessimistic about Joe SCSI device
    options         NCONS=2                 #1 virtual consoles
    options         USERCONFIG              #Allow user configuration with -c XXX
    
    config          kernel  root on da0 swap on da0 and da1 dumps on da0
    
    device          isa0
    device          pci0
    
    device          fdc0    at isa? port "IO_FD1" bio irq 6 drq 2 vector fdintr
    device          fd0 at fdc0 drive 0
    
    device          ncr0
    
    device          scbus0
    
    device          sc0 at isa? port "IO_KBD" tty irq 1 vector scintr
    device          npx0    at isa? port "IO_NPX" irq 13 vector npxintr
    
    device          da0
    device          da1
    device          da2
    
    device          sa0
    
    pseudo-device   loop            # required by INET
    pseudo-device   gzip            # Exec gzipped a.out's
    EOM
         exit 1
    fi
    
    cp -f /sys/compile/MINI/kernel /mnt
    
    gzip -c -best /sbin/init > /mnt/sbin/init
    gzip -c -best /sbin/fsck > /mnt/sbin/fsck
    gzip -c -best /sbin/mount > /mnt/sbin/mount
    gzip -c -best /sbin/halt > /mnt/sbin/halt
    gzip -c -best /sbin/restore > /mnt/sbin/restore
    
    gzip -c -best /bin/sh > /mnt/bin/sh
    gzip -c -best /bin/sync > /mnt/bin/sync
    
    cp /root/.profile /mnt/root
    
    cp -f /dev/MAKEDEV /mnt/dev
    chmod 755 /mnt/dev/MAKEDEV
    
    chmod 500 /mnt/sbin/init
    chmod 555 /mnt/sbin/fsck /mnt/sbin/mount /mnt/sbin/halt
    chmod 555 /mnt/bin/sh /mnt/bin/sync
    chmod 6555 /mnt/sbin/restore
    
    #
    # create the devices nodes
    #
    cd /mnt/dev
    ./MAKEDEV std
    ./MAKEDEV da0
    ./MAKEDEV da1
    ./MAKEDEV da2
    ./MAKEDEV sa0
    ./MAKEDEV pty0
    cd /
    
    #
    # create minimum file system table
    #
    cat > /mnt/etc/fstab <<EOM
    /dev/fd0a    /    ufs    rw  1  1
    EOM
    
    #
    # create minimum passwd file
    #
    cat > /mnt/etc/passwd <<EOM
    root:*:0:0:Charlie &:/root:/bin/sh
    EOM
    
    cat > /mnt/etc/master.passwd <<EOM
    root::0:0::0:0:Charlie &:/root:/bin/sh
    EOM
    
    chmod 600 /mnt/etc/master.passwd
    chmod 644 /mnt/etc/passwd
    /usr/sbin/pwd_mkdb -d/mnt/etc /mnt/etc/master.passwd
    
    #
    # umount the floppy and inform the user
    #
    /sbin/umount /mnt
    echo "The floppy has been unmounted and is now ready."