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Dump/Restore Ultra-Mini-FAQ

Document v1.1

Disclaimer: I am not an expert in dump/restore.  In 
fact, I'm a newbie.  But I've been picking things up as
I implement it here and I wanted to pass some of those
things along in the form of a very basic FAQ.

-Patrick Walsh

1) Introduction/ Non-rewinding device
2) Dump command line
3) Sending 2 or more filesystems to a tape
4) Compressing dumps on the fly
5) The "nodump" file and directory attribute.
6) Restoring your dumps (including compressed).
7) How to confirm a backup (highly recommended)
8) What are the best buffer size options?
9) Experiencing bread, lseek, lseek2 errors.
10) Example backup script

 1) Introduction/ Non-rewinding device

You use dump to backup to a file or a tape device.  If
you're backing up to a tape device, then the first 
thing you need to understand is that there are two 
devices that refer to your tape drive.  There is the 
"rewinding" device and the "non-rewinding" device.

I wish I could tell you an easy way to figure out what
your device names are, but I don't know one.  On my
local box I had a /dev/tape device that linked to
/dev/st0.  It turns out that /dev/st0 is my "rewinding"
tape drive.  If I write to this device it will always
rewind before starting to write.  This means that if
you try to dump two filesystems, only the second one
will be stored.  If your tape device is /dev/st0, like
mine, then your non-rewinding tape device is probably
/dev/nst0.

Anyway, through the rest of this I will refer to $TAPE
and $RWTAPE.  $TAPE is the non-rewinding device (in my
case /dev/nst0) and $RWTAPE is the rewinding tape (in 
my case /dev/st0 and /dev/tape).  $FS is the filesystem
you are backing up, such as /dev/hda1.

 2) What options should I use?

Use the man page to figure out what options to send
to dump.  I use "dump 0uanf $TAPE $FS".

  u=update /etc/dumpdates after a successful dump
  a=auto-size -- bypass all tape length calculations 
    and write until eof
  n=notify 'operators' group when dump needs attention
  f=backup to file or device specified, or - for stdout

 3) You want to send two or more filesystems to tape.

OK, rewind using the mt command, then dump multiple
times to the non-rewinding device, and you're done:

mt -f $TAPE rewind
dump 0uanf $TAPE $FS1
dump 0uanf $TAPE $FS2
etc.

Check the man page of mt if you want to know how to
eject the tape or retension it or anything.

 4) You want to compress your dumps on the fly.  No
problem.  Send your backup to STDOUT and manipulate it
from there.  It's easier if you're sending your output
to the hard drive:

dump 0uanf - $FS | gzip -c > /backup/outfile.dump.gz

You want that to be written to the tape on the fly?  
Try this:

mt -f $TAPE rewind
dump 0uanf - $FS |gzip -c |dd if=- of=$TAPE

[ You can also use the -z or -J options of dump in the 
  recent versions to enable internal compression - stelian ]

 5) You read the man page and you're wondering what the
heck a "nodump" flag is.  For example, how can you get
dump to stop backing up /tmp or ~/.netscape/cache.  You
have two options: either exclude the inode in your dump
command, or flag the files and directories with the
"nodump" flag.  To flag /tmp, for example, do this:

chattr -R +d /tmp

Want more details?  Try 'man chattr' and 'man lsattr'.

 6) You want to know how to restore your backup.

Read the restore man page.  But barring that, the easy 
way is to use restore in interactive mode.  If you have
three filesystems on one tape and you want to restore 
files from the second one, you need to do this:

mt -f $TAPE rewind
mt -f $TAPE fsf 1     # skip forward one file
restore -if $TAPE

OK, suppose now that you used the commands in section 4
to compress the dump file before it was written to 
disk.  Use this command:

mt -f $TAPE rewind
mt -f $TAPE fsf 1
dd if=$TAPE of=- |gzip -dc |restore -rf -

Obviously if you dumped to a file instead of a tape it 
is much easier:

gzip -dc $filename |restore -rf -

 7) How to confirm your backup

 Check out the restore man page and read up on the -C 
option.

 8) What are the best buffer size options?
 
  Bernhard R. Erdmann answered this question on the 
dump-users mailing list.  His excellent response 
follows:

>> While I was doing google searches, there seems to be
>> an issue regarding the default buffer size writing 
>> to tape. According to some, the default buffer size 
>> of 512 can harm modern drives.  I have a Seagate 
>> DDS3 unit and am wondering what the best mt/dump 
>> options are.  I am using datacompression in 
>> hardware.  Should I go with a large buffer (mt 
>> setblk 10240) or variable (mt setblk 0).  If I 
>> change these sizes, does dump need to know about it?
>
>Dump/restore uses a default blocksize of 10 KB. You 
>can change it with the "-b" option or use dd for 
>(re-)blocking.
>
>dump 0ab 32 /
>dump 0af - / | dd obs=32k of=$TAPE
>ssh host "/sbin/dump 0af - /" | dd obs=32k of=$TAPE
>restore rb 32
>dd ibs=32k if=$TAPE | restore rf -
>ssh host "dd if=/dev/nst0 ibs=32k" | restore rf -
>
>Personally, I'd stick with variable blocksize on the 
>drive and use 32 KB as the application's blocksize as 
>Amanda uses that size, too.
>
>Rumours told using 64 KB or 128 KB blocksize yields to
>increase performance (maybe on the mtx list from an 
>Arkeia developer) didn't had any effects in my own 
>tests with blocksizes ranging from 10 to 64 KB some 
>months ago on a DDS-2 drive.
>
>Regarding tape drive performance at different block 
>sizes you may want to read 
>http://www.rs6000.ibm.com/support/micro/tapewhdr.html#Header_133
>
>- Block size, can effect the time for backup/restore. 
>Using large blocksizes may improve performance. Using 
>small block sizes can increase system overhead but 
>before changing to a large blocksize it is necessary 
>to be sure the user application supports the larger 
>blocksize chosen. 
>- Very long restore times due to blocksize. If a 
>backup is done with a fixed block length then the 
>restore should be done with the same fixed block 
>length. If a backup is done with a fixed block length 
>and the restore is done with variable block length, 
>the restore may work successfully but it may take many
>more hours to restore than it took to back up the 
>data. The reason for this is that when AIX reads fixed
>block length data in variable block mode, a check 
>condition is issued by the tape drive on every read. 
>AIX must interpret every check condition and determine
>the proper action to take. This often will put the 
>tape drive into a mode of reading that will require 
>the tape drive to stop tape motion, rewind the tape 
>some distance, then start reading again. This will 
>reduce the life expectancy of the tape and increase 
>the time it takes to backup data.

 9) Experiencing bread, lseek, and lseek2 errors.
 
  These errors are caused by inodes being changed
during the backup.  This is normal because dump and
the Linux kernel are both acessing the filesystem
and there is no consistence checking. By "calming"
the system (killing unnecessary processes), you 
decrease the likelihood of having these
errors.  Read up on the bug on Sourcefourge:

http://sourceforge.net/tracker/index.php?func=detail&aid=204909&group_id=1306&atid=101306

Note that the only "real" solution to this problem
is to dump a unmounted filesystem or use a filesystem
snapshot feature (like in LVM).

 10) That about sums up my knowledge on the matter, but
I feel better having written something for other peopleto look at so it doesn't take them quite so long to 
learn the things I did.  I've included my backup script
below.  There are much better ones floating around, so 
go find someone else's and use theirs if mine won't 
work for you or you don't understand it.


#!/bin/csh
# System backup script for NARNIA

# This is a script that will backup the entire hard drive
# to the NT server (not my choice) \\fs1.
#
# On each Sunday night, a full backup will be made
# of the hard drive and each day of the week thereafter an incremental
# backup will be made that captures only those changes since the night
# before.
# Each full backup will be sent to the local tape as well as to the
# NT machine.
#
# The files will be stored in partition-specific files with integer
# endings that specify the day of the week they were saved.  Files
# with zero on the end will always be full backups.

# Dump options:
#   a=auto-size -- bypass all tape length calculations and write until eof
#   f=write the backup to file or device specified or - for stdout
#   n=notify operators group when dump needs attention
#   u=update /etc/dumpdates after a successful dump

# Set variables that control the script.
setenv MOUNTPOINT '/root/fs1backup'
setenv OUTDIR '/root/fs1backup/narnia'
setenv TAPE '/dev/nst0'  # non-rewinding tape

# Auto-set variable that determines level of backup.
setenv DAY `date +'%w'`

# Mount the backup partition to /root/fs1backup
/usr/bin/smbmount \\\\fs1\\backup $MOUNTPOINT -o
"username=uname,password=pword"

# Delete files created on this day last week
rm -f $OUTDIR/*$DAY.dump.gz

# Do the actual backing up, one filesystem at a time.

# /dev/hda1 = /boot
/sbin/dump $DAY'uanf' - /dev/hda1 | gzip -c >$OUTDIR/boot-$DAY.dump.gz

# /dev/hda2 = /
/sbin/dump $DAY'uanf' - /dev/hda2 | gzip -c >$OUTDIR/root-$DAY.dump.gz

# /dev/hda3 = /usr
/sbin/dump $DAY'uanf' - /dev/hda3 | gzip -c >$OUTDIR/usr-$DAY.dump.gz

# /dev/hdb2 = /u1
/sbin/dump $DAY'uanf' - /dev/hdb2 | gzip -c >$OUTDIR/u1-$DAY.dump.gz


# OK, presumably everything is now backed up to \\fs1. On level 0
# dumps, lets backup to the local drive too.
if ($DAY == 0) then
 mt -f $TAPE retension
 foreach i ($OUTDIR/*0.dump.gz)
  dd if=$i of=$TAPE
 end
 mt -f $TAPE rewind
endif


# Unmount the backup partition, not needed outside of script
umount /root/fs1backup

# Explicitly free up the temporary variables
unsetenv DAY
unsetenv MOUNTPOINT
unsetenv OUTDIR
unsetenv TAPE


# RESTORE DIRECTIONS:
# If from tape:
#    dd if=$TAPE of=- | gzip -dc | restore -rf -
# or dd if=$TAPE |gzip -dc |restore -rf -
#
# Note: must queue tape to proper position first.  This
# is done by first rewinding then advancing to the proper
# file.  The order that files are written to tape is
# *probably* 0=/boot 1=/ 2=/usr 3=/u1
#
# Use mt to skip between them:
#  mt -f $TAPE rewind
# restore -if $TAPE   #now restoring /boot, probably
# mt -f $TAPE fsf 1
# restore -if $TAPE   #now restoring /
# mt -f $TAPE fsf 1
# restore -if $TAPE   #now restoring /usr
# #etc.
#
# Otherwise:
#    gzip -dc $filename | restore -rf -