19.12. Backup Basics
Προηγ | Κεφάλαιο 19. Αποθηκευτικά Μέσα | Επόμενο

## 19.12. Backup Basics

The three major backup programs are dump(8), tar(1), and cpio(1).

### 19.12.1. 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.

### Σημείωση:

If you use `dump` on your root directory, you would 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 the `.rhosts` file on the remote computer. The arguments to `rdump` and `rrestore` must be suitable to use on the remote computer. When `rdump`ing from a FreeBSD computer to an Exabyte tape drive connected to a Sun called `komodo`, use:

`#` `/sbin/rdump 0dsbfu 54000 13000 126 komodo:/dev/nsa8 /dev/da0a 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`.

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 using `dump`'s built-in method, setting the environment variable `RSH`:

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

### 19.12.2. `tar`

tar(1) also dates back to Version 6 of AT&T UNIX (circa 1975). `tar` operates in cooperation with the file system; it 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.

On FreeBSD 5.3 and later, both GNU `tar` and the default `bsdtar` are available. The GNU version can be invoked with `gtar`. It 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/gtar cf komodo:/dev/nsa8 . 2>&1`

The same could be accomplished with `bsdtar` by using 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`.

### 19.12.3. `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` 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`.

`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/nsa0`).

### 19.12.4. `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`.

### 19.12.5. 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 &#x201C;holding disk&#x201D; to backup several file systems at the same time. Amanda creates &#x201C;archive sets&#x201D;: 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 &#x201C;archive set&#x201D; 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.

### 19.12.6. Do Nothing

&#x201C;Do nothing&#x201D; 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 &#x201C;Do nothing&#x201D; 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.

&#x201C;Do nothing&#x201D; 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.

### 19.12.7. 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.

### 19.12.8. Emergency Restore Procedure

#### 19.12.8.1. Before the Disaster

There are only four steps that you need to perform in preparation for any disaster that may occur.

First, print the bsdlabel from each of your disks (e.g. `bsdlabel da0 | lpr`), your file system table (`/etc/fstab`) and all boot messages, two copies of each.

Second, determine that the boot and fix-it floppies (`boot.flp` and `fixit.flp`) have all your devices. The easiest way to check is to reboot your machine with the boot floppy in the floppy drive and check the boot messages. If all your devices are listed and functional, skip on to step three.

Otherwise, you have to create two custom bootable floppies which have a kernel that can mount all of your disks and access your tape drive. These floppies must contain: `fdisk`, `bsdlabel`, `newfs`, `mount`, and whichever backup program you use. These programs must be statically linked. If you use `dump`, the floppy must contain `restore`.

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

Fourth, test the floppies (either `boot.flp` and `fixit.flp` or the two custom bootable floppies you made in step two.) and backup tapes. Make notes of the procedure. Store these notes with the bootable floppy, the printouts and the backup tapes. You will be so distraught when restoring that the notes may prevent you from destroying your backup tapes (How? In place of `tar xvf /dev/sa0`, you might accidentally type `tar cvf /dev/sa0` and over-write your backup tape).

For an added measure of security, make bootable floppies and two backup tapes each time. Store one of each at a remote location. A remote location is NOT the basement of the same office building. A number of firms in the World Trade Center learned this lesson the hard way. A remote location should be physically separated from your computers and disk drives by a significant distance.

Παράδειγμα 19.3. A Script for Creating a Bootable Floppy
#!/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 # bsdlabel -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 &lt;&lt; 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 &lt;&lt;EOM /dev/fd0a / ufs rw 1 1 EOM # # create minimum passwd file # cat > /mnt/etc/passwd &lt;&lt;EOM root:*:0:0:Charlie &amp;:/root:/bin/sh EOM cat > /mnt/etc/master.passwd &lt;&lt;EOM root::0:0::0:0:Charlie &amp;:/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."

#### 19.12.8.2. After the Disaster

The key question is: did your hardware survive? You have been doing regular backups so 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 your hardware is okay, check your floppies. If you are using a custom boot floppy, boot single-user (type `-s` at the `boot:` prompt). Skip the following paragraph.

If you are using the `boot.flp` and `fixit.flp` floppies, keep reading. Insert the `boot.flp` floppy in the first floppy drive and boot the computer. The original install menu will be displayed on the screen. Select the `Fixit--Repair mode with CDROM or floppy.` option. Insert the `fixit.flp` when prompted. `restore` and the other programs that you need are located in `/mnt2/rescue` (`/mnt2/stand` for FreeBSD versions older than 5.2).

Recover each file system separately.

Try to `mount` (e.g. `mount /dev/da0a /mnt`) the root partition of your first disk. If the bsdlabel was damaged, use `bsdlabel` to re-partition and label the disk to match the label that you printed and saved. Use `newfs` to re-create the file systems. Re-mount the root partition of the floppy read-write (`mount -u -o rw /mnt`). Use your backup program and backup tapes to recover the data for this file system (e.g. `restore vrf /dev/sa0`). Unmount the file system (e.g. `umount /mnt`). Repeat for each file system that was damaged.

Once your system is running, backup your data onto new tapes. Whatever caused the crash or data loss may strike again. Another hour spent now may save you from further distress later.

Προηγ | Πάνω | Επόμενο
19.11. Backup Strategies | Αρχή | 19.13. Network, Memory, and File-Backed File Systems

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