backup techniques
Post on 07-Jan-2016
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Backup Techniques• Objectives
– to illustrate practical application of backup utilities
• Contents– using dd, a direct device access command
– manipulating magnetic tapes
– getting best of cpio
– working with DOS format diskettes
– network backups
• Practical– to use DOS utilities
• Summary
Direct Device Access• Use dd to access a device directly
• Useful command parameters:of=file write to named file instead of stdout
if=file read from named file instead of stdin
bs=size specify block size (also ibs and obs)
count=n copy just n records
conv=ascii convert EBCDIC to ASCII
conv=ebcdic convert ASCII to EBCDIC
conv=ibm slightly different convert ASCII to EBCDIC
conv=swab swap every pair of bytes (big endian - little endian)
# dd if=/dev/nst0 of=/tmp/ibm.tape \bs=4095 conv=ibm,swab
# dd if=/dev/nst0 of=/tmp/ibm.tape \bs=4095 conv=ibm,swab
Exercise - Copying a Disk• How would you copy any format floppy disk?
• Checkout Linux support for msdos mimic commandsman –k mtools
mdir
mcopy
mformat
#change disks#
#change disks#
Using dd To Identify File Type• Imagine you are given a backup disk marked with file name
but no information about command used to create it– you are asked to restore data from that backup
– which utility will you use?
• This is where dd comes into it:
• This is much neater than trying each utility in turn to see if it works!
# dd if=/dev/fd0 count=1 of=test1read just one block from the device and store it in a file
# file test1test1: tar
# dd if=/dev/fd0 count=1 of=test1read just one block from the device and store it in a file
# file test1test1: tar
Tape Device Names• Tape devices are kept in /dev/
– device names are usually simple numbers
• By default tapes rewind after writing– tapes are not rewound before use but after
– device names ending or beginning in n will not rewind after use
• None SVR4 systems use other conventions– SCSI tape devices begin with st and nst
– Can be accessed in modes a, l, m for dencity
– Tapes are managed by mt command
# cd /# echo "Filesystem: /etc\nDate: `date`" | dd of=/dev/nst0# find etc -print | cpio -ocv >/dev/st0# dd if=/dev/nst0# cpio -itvc </dev/st0
# cd /# echo "Filesystem: /etc\nDate: `date`" | dd of=/dev/nst0# find etc -print | cpio -ocv >/dev/st0# dd if=/dev/nst0# cpio -itvc </dev/st0
Handling Tapes with mt• Manipulate magnetic tapes with the mt command
– specify the tape device with -f
• Useful command parameters recognised:rewind rewind to start of tape
offline rewind and take offline (drive may eject tape)
fsf n skip forward n files
bsf n skip backwards n files
eom skip to end of media
status print tape unit status information
tell tell where on tape
# cd /# mt -f /dev/st0 rewind# echo "Filesystem: /etc\nDate: `date`" | dd of=/dev/st0# find etc -print | cpio -ocv >/dev/st0# mt -f /dev/st0 rewind# mt -f /dev/st0 fsf 1# cpio -itvc </dev/nst0# mt -f /dev/st0 offline
# cd /# mt -f /dev/st0 rewind# echo "Filesystem: /etc\nDate: `date`" | dd of=/dev/st0# find etc -print | cpio -ocv >/dev/st0# mt -f /dev/st0 rewind# mt -f /dev/st0 fsf 1# cpio -itvc </dev/nst0# mt -f /dev/st0 offline
Linux filesystem dump• Not Recommended! (linux kernel 2.4)
• The dump and restore programs are Linux equivalents to the UNIX programs of the same name. As such, many system administrators with UNIX experience may feel that dump and restore are viable candidates for a good backup program under Red Hat Linux. Unfortunately, the design of the Linux kernel has moved ahead of dump's design.
# /sbin/dump -0u -f /dev/st0 /usr/src
# restore –i rf /dev/st0
# /sbin/dump -0u -f /dev/st0 /usr/src
# restore –i rf /dev/st0
Backup & Restore with tar• Most classic and secure of them all
– Minus, cannot backup empty directories– Operate on filesystem level– Filelocking can prevent backup of files
• Backup and restore with tar
• Use crontab to schedule backups14 15 * * 6 /bin/tar cvfz /home/usershare/school/text.tar.gz /home/ftp/pub/text/*
• Incremental backups
# tar cvf /dev/st0 /home
# cd /mnt/restored; tar xvf /dev/st0
# tar cvf /dev/st0 /home
# cd /mnt/restored; tar xvf /dev/st0
Prepairing the Backup script1. Create the backup script backup.cron file,
touch /etc/cron.daily/backup.cron - write the script on previous page and save it as backup.cron
2. date +%d%b < /backups/last-full/myserver-full-date
3. chmod 755 /etc/cron.daily/backup.cron
NOTE!
When restoring backups it is nice to have back permissions, not only the files.
tar xpf backup.tar
Up to 38% of the backup tapes will fail!
It takes as long time as the backup took to recover!
• Speed up backups and reduce storage using compression– use the Unix compress utility to reduce archive size
• Use dd to work with larger tapes blocks– can also use the -C option to cpio (SVR4)
• TAR is most popular backup method
• Not all tape modes might be supported
Putting Them Together
# find . -print | cpio -ocvB | \
compress | dd of=/dev/st0 bs=65536
# dd if=/dev/st0 bs=65536 | \
uncompress | cpio -itvcB
# find . -print | cpio -ocvB | \
compress | dd of=/dev/st0 bs=65536
# dd if=/dev/st0 bs=65536 | \
uncompress | cpio -itvcB
Network Backups• Use the rsh or rexec commands to run the backups across
the network
• Use NFS or similar to mount network filesystems– data can be backed up from any filesystem including networked drives
• If using network drives be careful about backing filesystems you don't want
– use the -mount option to find to restrict the file search
# find . -print | cpio -ocvB | rsh rosies dd of=/dev/st0
# rsh rosies dd if=/dev/st0 | cpio -itvcB
# find . -print | cpio -ocvB | rsh rosies dd of=/dev/st0
# rsh rosies dd if=/dev/st0 | cpio -itvcB
# rsh mash4077 'find . -print | cpio -ocvB' | dd of=/dev/st0
# dd if=/dev/st0 | rsh mash4077 cpio -itvcB
# rsh mash4077 'find . -print | cpio -ocvB' | dd of=/dev/st0
# dd if=/dev/st0 | rsh mash4077 cpio -itvcB
Summary• Use dd for direct device access• dd can be used to encompass and read any
format backup • Tapes can be controlled with mt command• Unix support special commands for working
with DOS format diskettes• Backup utilities can be used in conjunction
with compress command to save storage• Using combination of rsh command and
backup utilities we can perform backups over the network
• Avoid dump and restore• Use TAR or CPIO• Use script for scheduling backups
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