Week 9 - Managing Backup and Recovery

Question 1

System backup

Answer 1

Copies files and directories to an archive

Use to restore data in case of system failure of data loss and corruption

Most common backup medium is tape

Question 2

Archive

Answer 2

File containing many other files, each of which is still identified by its file name, owner, permissions, and timestamp

Question 3

System backups in Linux

Directories to include in regular backups

Answer 3

/etc contains core configuration files, security files, network configuration files, user and group
information, etc

/home each user has a /home directory

/opt software and packages added after the default installation

/root root user’s home directory

/var system specific information that changes while the system is running normally

/srv server scripts

Question 4

Tar Utility

Answer 4

Tar (tape archive) utility

Archives files
- creates archives by combining multiple files and directories into a single file
- originally designed to store files on magnetic tape
- now used to store files on a tape, hard disk, CD/DVD, or even on a network

Extracts files
- unpack members of an archive making them available to the file system

Question 5

Common tar options

Answer 5

-c or –create
creates an archive file

-t or –list
Lists an archive’s contents

-x or –extract
Extracts an archive’s contents

-f or –file
Specifies the archive’s file name and location

-v or –verbose
Displays details about copying files to and extracting files from archives

-z or –gzip, –unzip
Filters an archive through gzip

Question 6

Members

Answer 6

– Different from files
– Can be viewed only with the tar command’s t option
– Can extract all/some members from an archive
* e.g. When a user deletes a file accidentally

Question 7

• r or –append

Answer 7

Adds files to existing archive

Question 8

-u or –update

Answer 8

Compares the date and time of a member with the date and time of the file with the same name. If the file was modified after the archive was created, its newer version is added to the archive.

Question 9

-A or –concatenate

Answer 9

Similar to append option but adds one archive to another archive

Question 10

–delete

Answer 10

removes specific members from a tar archive. This option doesn’t have a short name

Question 11

-d or –compare

Answer 11

compares specified members with files in the file system having the same name and reports difference in file size, mode, owner, modification date and contents

Question 12

–mode

Answer 12

changes a member’s permissions in an archive with number or symbolic notation

Question 13

–mtime

Answer 13

changes a member’s modification date in the tar archive

Question 14

–owner

Answer 14

changes the member’s owner in the tar archive

Question 15

–group

Answer 15

similar to –owner, except it’s user to change member’s group instead of owner

Question 16

Full backup

Answer 16

Archive of all files on file system

Never perform a full backup while users are accessing the system - tar file could be corrupted if files are modified during backup process

Question 17

tar command for full backup

Answer 17

tar -cvf backup0.tar -V “Comment” –listed-incremental =/home/backup.snap *

creates full backup of current directory

-cvf creates archive called backup0.tar
-v labels archive with the text between quotation marks
–listed-incremental creates a snapshot file named backup.snap which is used during incremental backups to determine which files have changed since the last backup
* what files to backup

Question 18

Incremental backups

Answer 18

Archive containing only files modified since the last backup

– Companies have different backup strategies,
depending on their needs
– Incremental backup each night, full backup weekend
– Full Backup each night

• Syntax for creating an incremental backup
  – Same as the command for full backups
  – If the snapshot file already exists, the tar utility
  examines it to determine whether any files have
  changed

Question 19

-V or –label

Answer 19

Adds volume header to tar file. Volume header is a digital label used to describe file. View volume header with -t option

Question 20

-w or –verify

Answer 20

verifies files have been included in archive correctly

Question 21

-g or –listed-incremental

Answer 21

Forces tar to archive only files that have been modified since last backup (full or incremental) by analysing a snapshot file.
The snapshot file is given as an argument to the –listed-incremental option so that tar utility knows which files have been changed, added or deleted since last backup.

Question 22

–no-check-device

Question 23

–no-check-device

Answer 23

When a device number changes (can happen when upgrading kernel version) an incremental backup might backup files that haven’t been changed. This option prevents this from happening by forcing tar utility to not rely on device number when preparing for incremental backup.

Question 24

–check-device

Answer 24

default option; necessary only when you want to undo –no-check-device option

Question 25

Cpio Utility

Answer 25

Copy in/out utility
- Uses results of ls or find command to generate files to be archived

Operates in 3 modes
- Copy-out, cpio created an archive from the output of ls or find commands
-Copy-in, extracts files from an archive
-Copy-pass, copies files from one directory to another

Question 26

Input redirection

Answer 26

Default standard input - keyboard
< symbol redirects input instead of getting input from keyboard, can get input from file

Question 27

Output redirection

Answer 27

default standard output, screen e.g. type ls and result is output to screen

Output redirection operators redirect command’s output
> redirects output to a file

> file redirection
| pipe redirection

redirects output of one command and sends it ass input to another commands

Question 28

ls > myfile

Answer 28

redirect ls command’s output to the myfile instead of displaying the output on screen

Question 29

ls | more

Answer 29

redirect ls output to more command

Question 30

Copy-Out Mode

Answer 30

cpio - copy in/out utility
Uses results of ls or find command to generate files to be archived

-o or –create
creates archives by accepting output of ls or find as input for an archive

e.g. ls | cpio -o > files.cpio
results of ls command used to determine which files to archive in files.cpio

Question 31

Copy-In Mode

Answer 31

Extract an archive
use standard input redirection symbol < to extract the archive members

-i extracts fies from a cpio archive

-v lists files as they are being extracted

Question 32

Copy-Pass Mode

Answer 32

Copy files and directories from one directory and paste them in another directory
- without actually creating an archive

Not practical option for option for backups
- Does almost same as cp command

Cpio utility
- Preserves modification times and ownership

-p copy files from one directory tree to another

Question 33

Compression

Answer 33

Reduces size of data to store information in less space

Every GB of data costs money

Compress data to:
- Save storage space
- Make it fit on removable media
- Transfer it across network faster

Most common compression utility gzip
bzip2 also available

Question 34

The gzip Utility

Answer 34

File compressed with gzip has:
– Extension .gz
–Same file permissions, ownership, and modification time as the original file
– Much smaller file size

• Compression ratio
  – Defines by how much a file is reduced after compressing it
  – Text files are typically reduced by 60% to 70% with compression
  – -v option: view compression ratio

gzip is built into tar
- Use -z option to enable gzip
- Compress an archive as you create it

Question 35

Scheduling Backups

Answer 35

Configure Automatic system backups
– Backing up is a chore
– Management can often prioritize other work
– 75% of data loss is human error related
– Vital areas of software production, software deployment, data warehousing etc.

Question 36

CRON (Command Run ON)

Answer 36

Automatic scheduling and execution of system backups
– Schedule during off hour where fewer people are logged in to the system
– Less disruption in service
- Use CRON

Question 37

CRON daemon (crond)

Answer 37

System daemon
Uses a configuration file called a cron table to schedule commands that run at specified times

Cron table has 6 fields
-Fields used to specify time to run task and task to be run

• • • • • command to be executed
          minute (0-59)
          hour (0-23)
          day of month (1-31)
          month (1-12)
          day of week (0-6) Sunday = 0

Question 38

Cron daemon uses two types of cron tables

Answer 38

– User cron table in /var/spool/cron/tabs
– System cron table in /etc/crontab
* Users use the user cron table to schedule tasks
* System uses the system cron table to schedule system tasks

Question 39

User Cron tables

Answer 39

Every user (including root) on an Linux system has a cron table
* crontab command
– Create, delete, and list cron tables

Question 40

crontab command options

Answer 40

-e opens vim editor to edit current user’s cron table. If cron table doesn’t exist, a blank table is created for you to edit.
-u specifes name of user whose cron table is to be edited
-l displays current user’s cron table
-r removes current user’s cron table

Question 41

Displaying contents of text file

Answer 41

List a file’s contents without actually opening file in text editor
cat and tic
head and tail
more and less

Question 42

The cat and tic commands

Answer 42

cat (concatenation) command
– Displays an entire file’s contents at once
– Typically used to display the contents of a small text file
–Can be used to display the contents of multiple files at once
* -n option
– Display line numbers in a text file:
– cat -n scr1

tic command
– Display a text file’s contents in reverse order
– Main purpose to display log files

Question 43

The head and tail commands

Answer 43

head command
– Displays the first 10 lines of a text file
– head scr8
– Can display more than 10 lines: head 15 scr8

tail command
– Displays the last 10 lines of a text file
– tail scr8
– Can display more than 10 lines tail 15 scr8
+ operator
* Start displaying text at a specified line number all the way to the end of the file

Question 44

The more and less commands

Answer 44

more command
– Displays a file’s contents one screen at a time

• less command
  – Displays a file’s contents one screen at a time
  – Allows you to navigate the file by using arrow keys or the mouse wheel

Spacebar Displays next bar
#+spacebar Display the next # lines
Enter Displays next line
q Exits the more command
= Displays current line number
h Displays help

Question 45

Summary

Answer 45

Several directories should be backed up regularly particularly user’s home directories
Linux includes backup utilities tar and cpio
Archives stored on many Stored on many different types of media, such as tapes, CD RWs/DVD RWs, removable media, and hard disks