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Data Structures

Backing up and Archiving DataBacking up and Archiving Data

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Learning ObjectivesLearning Objectives

Explain backing up.

Discuss sensible systems for managing back ups.

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Backing upBacking up

Making regular multiple copies (at least one Making regular multiple copies (at least one copy should be kept in a different location to the copy should be kept in a different location to the original) of an entire data file / all the records in original) of an entire data file / all the records in a file on to a different portable medium for short a file on to a different portable medium for short term storage together with a transaction log, in term storage together with a transaction log, in case the original data is corrupted due to:case the original data is corrupted due to: A system malfunction. A system malfunction. Viruses.Viruses. Accidental deletion.Accidental deletion.

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Why back up?Why back up?

Data stored in files is very valuable. Data stored in files is very valuable. It has taken a long time to input to the system, and It has taken a long time to input to the system, and

often, is irreplaceable. often, is irreplaceable. If a bank loses the file of customer accounts because the If a bank loses the file of customer accounts because the hard disk crashes, then the bank is out of business.hard disk crashes, then the bank is out of business.

There should be more than one back-up copy of There should be more than one back-up copy of a file:a file: Some of these copies will be stored away from the Some of these copies will be stored away from the

computer system in case of something like a fire computer system in case of something like a fire which would destroy everything in the building.which would destroy everything in the building.

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How often should files be backed up?How often should files be backed up?

There are no right answers, but there are wrong There are no right answers, but there are wrong ones. ones. It all depends on the application. It all depends on the application. An application that involves the file being altered on a An application that involves the file being altered on a

regular basis will need to be backed up more often regular basis will need to be backed up more often than one that is very rarely changed (what is the point than one that is very rarely changed (what is the point of making another copy if it hasn’t changed since the of making another copy if it hasn’t changed since the previous copy was made?). previous copy was made?).

A school pupil file may be backed up once a A school pupil file may be backed up once a week, whereas a bank customer file may be week, whereas a bank customer file may be backed up hourly.backed up hourly.

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Backup ProblemBackup Problem

If a back up is made at 9.00am and an If a back up is made at 9.00am and an alteration is made to the file at 9.05am, if alteration is made to the file at 9.05am, if the file now crashes, the back up will not the file now crashes, the back up will not include the change that has been made. include the change that has been made.

It is very nearly the same, but not quite. It is very nearly the same, but not quite.

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Transaction LogTransaction Log

A separate file of all the changes that have been A separate file of all the changes that have been made since the last back up is kept. made since the last back up is kept.

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Transaction LogTransaction Log

A separate file of all the changes that have been A separate file of all the changes that have been made since the last back up is kept. made since the last back up is kept. Needed because a back-up copy will rarely be the Needed because a back-up copy will rarely be the

same as the original file because the original file same as the original file because the original file keeps changing:keeps changing:

A transaction log is very rarely used: A transaction log is very rarely used: Once a new back up is made the old transaction log can be Once a new back up is made the old transaction log can be destroyed. destroyed.

Speed of access to the data on the transaction log is not Speed of access to the data on the transaction log is not important because it is rarely used, so a transaction log tends important because it is rarely used, so a transaction log tends to use serial storage of the data and is the best example of a to use serial storage of the data and is the best example of a serial file if an examination question asks for one.serial file if an examination question asks for one.

Disk Mirroring

A technique used to protect a computer system A technique used to protect a computer system from loss of data and other potential losses due from loss of data and other potential losses due to disk failures. to disk failures.

The data is duplicated by being written to two or The data is duplicated by being written to two or more identical hard drives, all of which are more identical hard drives, all of which are connected to one disk controller card. If one connected to one disk controller card. If one hard drive fails, the data can be retrieved from hard drive fails, the data can be retrieved from the other mirrored hard drives. the other mirrored hard drives. Disk mirroring if often referred to as RAID 1 or RAID Disk mirroring if often referred to as RAID 1 or RAID

Level 1.Level 1.

9921/04/2321/04/23 https://www.techopedia.com/definition/25959/disk-mirroring

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A library keeps both a book file and A library keeps both a book file and a member file. a member file.

The library does a stock take twice a The library does a stock take twice a year and orders new books only once a year and orders new books only once a year. Members can join or cancel their year. Members can join or cancel their membership at any time. membership at any time.

a)a) Describe how the library can implement Describe how the library can implement a sensible system of backing up their a sensible system of backing up their files.files.

(4)(4)

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A library keeps both a book file and A library keeps both a book file and a member file. a member file.

Book file needs to backed up twice (or three) times a Book file needs to backed up twice (or three) times a year when the stock take or book purchasing has year when the stock take or book purchasing has made the file alter.made the file alter.Member file needs backing up daily (at least weekly) Member file needs backing up daily (at least weekly) because of constant changes.because of constant changes.A transaction log would also need to be kept for the A transaction log would also need to be kept for the member file.member file.Back up copies would be stored away from the building Back up copies would be stored away from the building with the computer system in it to ensure that a copy of with the computer system in it to ensure that a copy of files survived in case of fire.files survived in case of fire.Multiple copies of the book file would be made.Multiple copies of the book file would be made.Member file copies and transaction logs may be kept Member file copies and transaction logs may be kept for a number of back up periods.for a number of back up periods.

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PlenaryPlenary

What is backing up?

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PlenaryPlenary

Back up Making a copy of the data in a file, so that if Making a copy of the data in a file, so that if

the disk is destroyed, the data can be the disk is destroyed, the data can be recovered. recovered.