Backing Up 01/12/2018

Learning Objectives Explain backing up. Discuss sensible systems for managing back ups. 01/12/2018

Backing up Making regular multiple copies (at least one copy should be kept in a different location to the original) of an entire data file / all the records in a file on to a different portable medium for short term storage together with a transaction log, in case the original data is corrupted due to: A system malfunction. Viruses. Accidental deletion. 01/12/2018

Why back up? Data stored in files is very valuable. It has taken a long time to input to the system, and often, is irreplaceable. If a bank loses the file of customer accounts because the hard disk crashes, then the bank is out of business. There should be more than one back-up copy of a file: Some of these copies will be stored away from the computer system in case of something like a fire which would destroy everything in the building. 01/12/2018

How often should files be backed up? There are no right answers, but there are wrong ones. It all depends on the application. An application that involves the file being altered on a regular basis will need to be backed up more often than one that is very rarely changed (what is the point of making another copy if it hasn’t changed since the previous copy was made?). A school pupil file may be backed up once a week, whereas a bank customer file may be backed up hourly. 01/12/2018

Backup Problem If a back up is made at 9.00am and an alteration is made to the file at 9.05am, if the file now crashes, the back up will not include the change that has been made. It is very nearly the same, but not quite. 01/12/2018

Transaction Log A separate file of all the changes that have been made since the last back up is kept. 01/12/2018

Transaction Log A separate file of all the changes that have been made since the last back up is kept. Needed because a back-up copy will rarely be the same as the original file because the original file keeps changing: A transaction log is very rarely used: Once a new back up is made the old transaction log can be destroyed. Speed of access to the data on the transaction log is not 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 serial file if an examination question asks for one. 01/12/2018

Disk Mirroring A technique used to protect a computer system from loss of data and other potential losses due to disk failures. The data is duplicated by being written to two or more identical hard drives, all of which are connected to one disk controller card. If one hard drive fails, the data can be retrieved from the other mirrored hard drives. Disk mirroring if often referred to as RAID 1 or RAID Level 1. 01/12/2018 https://www.techopedia.com/definition/25959/disk-mirroring

A library keeps both a book file and a member file. The library does a stock take twice a year and orders new books only once a year. Members can join or cancel their membership at any time. a) Describe how the library can implement a sensible system of backing up their files. (4) 01/12/2018

A library keeps both a book file and a member file. Book file needs to backed up twice (or three) times a year when the stock take or book purchasing has made the file alter. Member file needs backing up daily (at least weekly) because of constant changes. A transaction log would also need to be kept for the member file. Back up copies would be stored away from the building with the computer system in it to ensure that a copy of files survived in case of fire. Multiple copies of the book file would be made. Member file copies and transaction logs may be kept for a number of back up periods. 01/12/2018

Plenary What is backing up? 01/12/2018

Plenary Back up Making a copy of the data in a file, so that if the disk is destroyed, the data can be recovered. 01/12/2018