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 Timesheet Father Defn: A large number of Transact Employee similar paper documents G34 master are typed up onto a Payroll transaction tape. The Week 34 transaction tape updates the master file at a regular set Father Timesheet time. Eg Payroll: Transact Employee timesheets are typed into a master G35 transaction file and update G35 Payroll 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 G36 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 Interactive 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 Account Some large volume data systems master 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! Digital 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 someone! 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 system 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 process? 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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