Backup Strategies - Fatmax by wuyunqing


									The aim of producing a backup strategy
is to make sure that a computer system
 can be returned to its original state if
     data has been lost or corrupted.
     Backup Strategy Recap
          ICT2 Ch19
A backup strategy is put in place to avoid permanent data loss
and to ensure the integrity of stored data ie can get back to a
previous version and build up the data correctly if current
data found to be in error.
Factors that should be considered in a backup strategy:
1. Selection of storage medium depends on volume of data
2. Frequency of backup depends on how often the data is updated
3. No. of backup copies kept depends how often data is updated
4. What will be copied; full (all data) or differential (just changes)
5. Recovery procedures; how quickly correct data is needed
6. Location of backup – how secure is this data?
         Recovery Procedures
          ICT2 Recap Ch19
A backup strategy needs a well documented and tested
recovery procedure to ensure backup data is restored properly

Factors that should be considered in a recovery procedure:
1. A well documented procedure that allocates roles and
   responsibilities to staff.
2. Availability of alternative hardware and accommodation if the
   originals have been destroyed.
3. Availability of original software such as the customised
   operating system and application programs!
4. That up-to-date backup data is readily available
Different types of system require different strategies
 Batch processing systems:                               Grand
 Defn: A large number of       Transact
 similar paper documents         G34
 are typed up onto a                          Payroll
 transaction tape. The                       Week 34
 transaction tape updates the
 master file at a regular set                            Father
 time. Eg Payroll:             Transact                 Employee
 timesheets are typed into a                             master
 transaction file and update                              G35
 the employee master file                    Week 35
 each Friday night and                                    Son
 produce wage cheques and                               Employee
 payslips.                    How would the Son
                              file be regenerated?       master
Different types of system require different strategies
 Online backup of small/medium                              Father /
 interactive transaction systems:                             Son
 Transactions are typed in and                             Customer
 processed immediately before the                          Account
 next transaction is entered. Eg                             master
 Paying a gas bill a business can’t         transaction
 afford to lose even a small amount         processing
 of data but can lose some
 operational time while data is
                                       If the system crashes
 restored from backup. There is no
                                       the son master file is   Daily
 prepared transaction tape so the
                                       restored from last     Transaction
 system creates a record on an
                                       nights father backup      Log
 online transaction log as each
                                       after adding the days
 transaction is typed in. The master
                                       transaction log to it.
 file is backed up every night.
Different types of system require different strategies
 Online backup of large
 important interactive systems:                               Customer
 Some large volume data systems
 are critical to the operation of a
 business eg A bank customer
 accounts file. The bank can’t                  Interactive
 afford to lose even a small amount             transaction
 of data or lose operational time               processing
 while data is restored from backup.
 So the backup must be taken after     Disk mirroring:
 each online transaction and written   storing identical     Customer
 to easily accessed backing storage    data on 2 different    Account
  disk.                               disks at virtually the mirror
                                       same time (RAID).
 This is called Disk Mirroring and     Disks are in
 is a type of RAID.                    different locations.
 Selection of appropriate backup medium
It’s not just size that matters but the way backing store is used too!
 Selection depends on:                      Disk             Audio
 The capacity (amount of data             mirroring           Tape
 stored on the tape/disc); the speed      (RAID)             (DAT)
 of transfer; and the cost of the
 medium should be considered.
 Type of medium
 1. Floppy discs – not appropriate for most companies as only holds
    1.44Mb of data (approx 144,000 characters of data)
 2. DAT is magnetic tape and holds 12-20GB with transfer speeds up
    to 150Mb per minute. OK for small companies who may not mind
    being without data for up to a day while the data is restored.
 3. Disk mirroring is fast and backup always up-to-date, but expensive
 Selection of appropriate backup drive
If tapes are used to back up data then they usually have to be
manually loaded into the tape drive before backup can take
place. This is OK if just using one backup tape as it can be
preloaded and the backup program automatically runs
overnight without anyone needing to be there.

However if there is a lot of data to back up then two or more
tapes will be needed. This requires someone to unload and load
them as the backup program runs – sleepless nights for

A tape library is a device that makes unattended large scale
backups possible. It consists of a tape drive and a robotic
device that can move tapes in and out of the drive.
Selection of Backup Location
Backup files must      1. Sometimes a fireproof safe is used to
be kept in a              store backups in the same building
separate location
                       2. Sometimes they are kept at another
to the original file
                          building owned by the company
in case of fire or
theft.                 3. In large cities firms exist that provide a
                          backup service. They come and collect
However if they
                          the backups daily and store at their own
are kept a long
                          premises. They also provide correctly
distance away the
                          labelled tapes. If disaster strikes they will
time needed to get
                          return the backups immediately.
the backups
increases the time     4. Remote backup – similar to no3 but
taken to restore          backup is transmitted over a WAN to the
files after a             firm. Data can be vulnerable and needs to
disaster.                 be compressed and encrypted.
         Scheduling Types of Backup
Full Backup – A full copy of the system is copied to tape each night. A
different tape is used each day. Restoration is easy as the system can be
restored from just one tape. However if the volume of data is large then
the backup may take many hours and might not be complete before the
next days business begins.
Differential Backup – One full copy of the system is copied to tape over
the weekend. Only changes to the data are copied to tape during the
week. This allows the data to be restored with a maximum of just 2 tapes.
It takes longer to restore the original data as the changes must be
applied to the full weekend backup.
Incremental Backup - One full copy of the system is copied to tape
over the weekend. Only changes to that days data are copied to tape
during the week. This allows the data to be restored with a maximum of 6
tapes. It takes much longer to restore the original data as each daily
tape must be applied in order to the full weekend backup. (However the
actual backup is faster as changes only copied once.)
        Tape rotation methods
It is bad practice to only use one backup tape – if this breaks?!….
A number of generations of backup will require careful labelling
and a tape rotation system is used to ensure that correct versions
are kept. (Legal obligations can dictate how long data is kept)
The Grandfather, Father, Son Method
If data is updated on a daily basis then: Tapes are labelled by the
day of the week with a different tape for each Friday in the month
and a different tape for each month of the year. (Assuming no data
changes on Sat or Sun) So Mon, Tue, Wed Thur tapes are
overwritten weekly and Fri tapes are overwritten monthly. This still
means there are over 20 generations of current backup tape for this
A written backup log should be kept of backups taken with date,
time, name of labelled tape, name of person who did backup.
Questions – how well have you listened!!
1. What is the main method of tape rotation?
2. What are the 3 types of scheduling backing up data?
3. What is a tape library?
4. How would you back up data from a batch process?
5. How would you back up data from a critical interactive transaction
6. How would you back up data from a non-critical interactive
   transaction process?
7. What are the advantages and disadvantages of backing up data
   remotely via a WAN?

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