File Organisation
INFORMATION PROCESSING
INFORMATION TECHNOLOGY
Introduction
When data has been input, verified and validated it
usually ends up stored in a file.
A file is a container in a computer system for storing
data, information and programs.
Files usually exist permanently on a secondary storage
media such as:
Disk drives (HDD and SSD)
USB pen drives
DVDs
File access methods
The term ‘file access methods’ can have two meanings in IT:
How data is stored in the file
How files are stored on the storage media
We need to be clear about the similarities and differences.
The important words are:
Serial
Sequential
Random and direct
File access methods
‘Serial access’ means to start at the beginning and read to
the end in one go.
Serial access – reading in the order in which the words were
written – is great for a short magazine but not so good for a
long book.
‘Sequential access’ requires the material to be ordered in
some way. The chapters in a book are in order so we can
access any chapter we want, more quickly than serial
access.
File access methods
Random and direct access allows you to go directly to the
chapter or material you are looking for. It is like using the
table of contents page to quickly turn to the right page.
How data is stored in the file
First we will look at the ideas of serial, sequential and random
access as they apply to how data is stored in the file.
Serial file access is where the data is stored in the file in the
order in which the data was written to the file.
For example, when you save and close a document you
have been working on, the words are stored in the file in the
order they appear on the page. When you open it up later,
the words are read back into memory in the same order.
Serial files are easy and efficient to use when the whole file
can be easily held in main memory (RAM).
How data is stored in the file
In sequential file access, data is stored in an ordered way or
sequence.
An example of sequential access in a file is storing a
telephone directory. The data is stored in the order of last
name. if the telephone directory is too large to fit in RAM,
then sequential file access is better than serial access.
Random access and direct access do not usually refer to
how data is stored in a file.
How files
are stored
on storage
media
Serial file access
Files are stored on a storage medium in a serial way when
they are stored in the order in which they were written.
Serial access is simple but very inefficient.
Serial access is usually only used on secondary storage
devices when there are a small number of files that are not
accessed very often.
System backup and archiving are examples of programs that
deal with a small number of files that are not accessed often
and so serial access may be appropriate.
Serial file access
Backup is the copying of files to a separate, removable
storage device so that they can be restored to the original
location if the original data is ever lost or destroyed.
A system backup is a copy of all the files on a computer
system that can be used to restore the whole computer after
a case of hardware failure or data loss.
Archiving is storing files that are accessed only infrequently,
and modified even less frequently, on another storage
device.
Sequential file access
Files are stored on storage media in a sequential way when
they are stored in some sort of sequence.
The books in a library are stored on shelves in order of their
Dewey Decimal Number. You use the ‘binary chop’ technique
to find a book, just like searching for a word in the dictionary or
a phone number in a directory.
The data in these files are stored in alphabetical sequence.
Magnetic tapes can store files sequentially, however, this type
of storage is largely outdated. Historically, sequential file storage
on magnetic tape was used for payroll applications and
archiving.
Random file access
Random access and direct access are often used
interchangeably but they are different.
Random access – the organization of data in main memory
(random access memory or RAM)
Direct access – the organization of files on a disk drive
In random access, data can be quickly accessed wherever it
occurs in main memory.
Direct file access
Direct access storage is secondary storage where each file has
a specific location or unique address in the storage, allowing it
to be accessed quickly.
Consider books in the library again
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