1. Local Area Networks, 3rd Edition David A. Stamper
Part 4: Installation and Management
Chapter 11
LAN Administration: Backup and Recovery

2. Chapter Preview Data backup Data recovery Problem detection
In this chapter you will study:
Data backup
Data recovery
Problem detection
Problem resolution
Diagnostic hardware and software
Disaster planning

3. Data Backup
A good LAN-administration policy must provide a method of data recovery to correct data problems.
Recovery
If data in a database becomes corrupted or a text file is accidentally deleted, an organization will want to restore the data to a usable state. Restoring from backup is almost always the preferred way to do this.
Archiving
Records, or other data, that you no longer regularly need can be archived; that is stored in another location than your hard drive.

4. Ways in which Data Can Be Corrupted
An application program with a logic bug can change data incorrectly.
A user can accidentally erase a file.A user can accidentally destroy a file by copying a new file over it.
A user can maliciously destroy a file or data in a file.
A system failure can leave the database in a state of partial update.
A disk failure can destroy data or render it inaccessible.
An undetected virus can erase or otherwise destroy data.
A bug in system software, such as the database management system, can cause data loss, unreliable results, or data corruption.

5. Types of Backups Incremental Backup Differential Backup
This type of backup, also called a partial backup, backs up all files that have changed since the preceding backup
With an incremental backup, only files with the archive bit set are backed up.
Differential Backup
A differential backup backs all files that have changed since the last full backup.
A differential backup will not reset the archive bit.

6. Comparison of Incremental and Differential Backups
Incremental Backup
Contents
Differential Backup
Contents
Day
Files Updated
Sunday
Monday
Tuesday
Wednesday
Thursday
Friday
Full backup completed
FileA, FileB, FileC
FileA, FileD
FileE
FileA, FielC, FileF
FileA
FileA, FileB, FileC
FileA, FileD
FileE
FileA, FielC, FileF
FileA
FileA, FileB, FileC
FileA, FileB, FileC, FileD
FileA, FileB, FileC, FileD, FileE
FileA, FileB, FileC, FileD, FileE, FileF
FileA, FileB, FileC,FileD, FileE, FileF

7. Backup Generations First Generation of Backups First Generation
Tape 1
Tape 2
Grandfather
Grandfather
Tape 2
Tape 3
Father
Father
Tape 3
Tape 4
Son
Son
Tape 4
Tape 1
Unused
Next to Be Used

8. A Sample Backup Retention Policy
Backup Policy
Daily
Weekly
End of year
Back up all files changed since the backup of the preceding day.
Make two copies; store one copy off-site.
Back up all files.
Back up all files as of midnight, December 31.
Back up all files as of midnight at the end of the fiscal year.
Retention Policy
Retain weekly backups and daily backups for 1 month.
Retain the first backup of each month for 1 year.
Retain the end-of-year backups for 5 years.

9. Backup Frequency
Static data should be backed up at least twice, and the two versions should be stored in separate locations.
Dynamic files must be backed up more often. Some installations do daily backups; others find that a weekly backup is adequate.
Failure rates and timeliness of recovery are factors to consider as well. If failure rates and the need to immediately return to operational status are high, then backups should be more frequent than if failure is uncommon and timeliness is not a major factor.

10. How and When to Make Backups
Because most of today’s LAN backup utilities do not provide data integrity protection for on-line backups, it is usually best to create the backups when data is not being modified.
In many situations, the LAN is not used at all over night. Unattended backups are possible through the backup software itself or through separate software utilities.
If a company’s software does not provide unattended backup, the LAN administrator may still be able to start the backup remotely.

11. Data Inconstancy During File Backup
Time
Record A-$1,000
Record X-$1,500
Backup Starts
Record A-$1,000
File 1 Backup
Completes
Transaction Starts
Record A-$500
Record X-$1,500
Record A-$500
Record X-$2,000
Transaction Ends
File 2 Backup
Begins
Record X-$2,000
Backup is inconsistent, Record A show a balance of $1,000 and Record X shows a balance of $2,000
Record A-$500
Record X-$2,000

12. Use of a Before Image in a Transaction
Images
Time
File 1
File 2
$1,000
$500
Begin
Transaction
1. Capture before image of
record being changed.
File 1
$1,000
2. Deduct $300 from record
and write new balance.
$700
3. Capture before image of
record being changed.
File 2
$500
4. Add $300 to balance
of record in File 2.
$800
End
Transaction
If for some reason the transaction cannot be completed, the before images are used to back up database changes that have been made and thus restore the integrity of the database.

13. Database After Images Time File 1 File 2 After Images $1,000 $500
Begin
Transaction
1. Capture after image of
record being changed.
File 1
$700
Transfer $300
from File 1 to
File 2.
2. Deduct $300 from record
and write new balance.
$700
3. Capture after image of
record being changed.
File 2
$800
4. Add $300 to balance
of record in File 2.
$800
End
Transaction
If a failure occurs that destroys data in File 1 and/or File 2, the current state of the database can be reconstructed through backup tapes and the after images of completed transactions. The backup is first restored, and then the after images are written in chronological order to bring the database forward to a current, consistent state.

14. Synchronization of Backups and the After-Image Audit Trail
Time
After-Image
Audit Trail
Record 1 File 3
Record 20 File 2
Record 2003 File1
Record 992 File 1 .
Backup begins.
Current after-image audit trial is closed.
After-Image
Audit Trail
Backup
Tape
Record 123 File 4
Record 1209 File 1
Record File 1
Record File 3 .
New after-image audit trail started. The new after-image audit trail is synchronized with the backup just completed
Backup
Tape
Backup completes.

15. Recovery Steps Identify and correct the source of the problem.
Back up the data that has been corrupted.
Restore the most recent, valid backup version of the lost data.
Bring the data forward I time until it is both consistent and current.
Run diagnostic tests to ensure that the recovery has indeed corrected the problem and that the data is consistent.
Document the problem experience, corrective actions, and problems encountered during recovery.
Create and implement procedures to prevent similar future occurrences.

16. Disaster Planning
A disaster plan addresses situations that disable major portions of the system. Disaster planning covers situations arising from fires, earthquakes, floods, and intentional acts of system destruction.
A key aspect of a disaster plan is off-site data storage and hardware replacement.
After planning for software and data recovery, the LAN administrator should also have plans for rebuilding the system. This may include items such as sources of compatible hardware, identification of hardware at other corporate locations that might be borrowed for emergency use, identification of alternative locations for temporarily or permanently installing the new system, and identification of companies specializing in all aspects of LAN hardware, software, cabling, installation, and data recovery.

17. Items Included in Disaster Plan
Insurance
Software
Data
Amount of insurance coverage for software, hardware, and cabling
Insurance carrier
Steps required to begin replacing/repairing insured components
Location of off-site storage of software
Currency of off-site software backups
Device used to create off-site storage
Sources of replacement software
Companies specializing in recovering data from damaged media (backup tapes and disk drives)
Location of off-site storage of data
Currency of off-site data backups
Device used to create off-site data storage
Methods to bring off-site data forward to a current status

18. Items Included in Disaster Plan (cont.)
Hardware
Environment
Outside Help
Workstation configurations
Server configurations
LAN topology/wiring diagrams
Sources of replacement hardware
Sources for repairing broken hardware
Location of spare hardware
Alternative locations for establishing a new network environment
Minimum requirements for establishing a new network environment
Names of companies specializing in data recovery, setting up a new network, data entry, cabling repair, and so on