Information Processing Lecture 9B Criteria for File Organisation

Objectives After studying this week’s work you should be able to: know and be able to work with the criteria for the selection of file organisation methods relate these criteria to specific information processing uses describe the difference between relative and physical addressing know the different types of file used in updating files and batch processing.

Selection of File Organisation Methods In the previous lecture we looked at the different ways in which we could organise information. Here we examine some of the criteria we use for selecting from these methods for a particular application.

Criteria There are four main criteria : File Use Ratio File Volatility File Size User Requirements These criteria will be explored over the next few slides

File Use Ratio This ratio is calculated for a specific time period, e.g. a day, week or month. It is calculated by the formula: File Use Ratio = number of records accessed total number of records

Calculating the Ratio Suppose a file has 5000 records. Further, suppose we have examined the files and found that during January 600 records were accessed File Use Ratio for January = =0.12 This figure represents a relatively low use

What does the ratio mean? Less than 0.25 represents low use Between 0.25 and 0.75 would be medium use Greater than 0.75 would be high use It must be remembered that these usages would be based on a particular time-period.

Some Examples Low Use: Finding owner’s details in a Police database of a car used in a robbery Medium Use Updating customer “loyalty card” bonus after a sale High Use Processing of examination results on a database of students

File Volatility The volatility of the file is a broad indication of the extent to which the file needs modifying and updating Files with high volatility contain records which are updated almost constantly, for example bank account details Files with low volatility contain relatively stable data, for example archived data on student performance in examinations

Volatility and Indexing File indexing is only normally used where volatility is low In high volatility files, the processing cost of the index outweighs its advantages.

File Size The size of the file is an important criterion in deciding which method of organisation to use Accessing larger files using serial and sequential file techniques can cause processing times to lengthen excessively Very large files will require indexed or direct access.

User Requirements The main factor here is how the user wishes to access the file. There are two main types of use: Batch access where the user wishes to update or modify many files at one time, for example overnight processing of records. Interactive access where the files need to be accessed and updated constantly, and records need to be kept up-to-the minute.

Matching Methods to Requirements Sequential or Serial files are well-suited to Batch Access For interactive Access, Indexed or Direct files would normally be required

Further Criteria In addition to the four main criteria discussed so far, we also need to consider: The type of storage available The complexity of the data The availability and cost of software to carry out the tasks.

Addressing There are two different ways in which we can index the data for later retrieval: Physical Addressing Relative Addressing

Physical Addressing All addressing ultimately must address the physical location in which the data is stored. On a magnetic disk we would need to know : Cylinder Surface Sector

Cylinders, Surfaces and Sectors A Hard Drive is split up into separate disks called platters These can have one or two surfaces These are divided up into tracks and sectors

Cylinders A Cylinder is a collection of the equivalent tracks on the surfaces of different platters Surface 1 Track 2 Surface 2 Track 2 Surface 4 Track 2 Surface 3 Track 2

Example of Physical Addressing Record number A stored at cylinder 2 surface 3 sector 5

Relative Addressing Most file techniques use some form of relative addressing. This means that the address is the position in the file and not its physical location on tape or disk.

Example of Relative Addressing Relative AddressRecord____ A B B C etc...

Updating Files An information system consists of 6 types of file: Master File The set of permanent records Transaction File Records of changes, deletions etc. made to the master file Table File tables of static data used by other processes and files Report File Information used for display or printing Control File the file handling records History File Backup files

Batch Processing In batch processing, data is stored up to be processed at one event, usually at the end of the day’s workings Batch processing usually is a “single task” which continues until the job is finished.

Follow-Up Work We have developed all the ideas and techniques in these two lectures to allow you to develop an understanding of files and file handling. To follow this up, Rread The File Handling IP Booklet Read the Set Book pp

Summary Selection of a suitable method of file organisation depends on file use ratio, volatility, file size, user requirements File addresses can refer to physical locations or relative positions File updating requires the use of master, transaction, table, report, control and history files Batch Processing involves different techniques depending upon the file organisation method.