Development and Implementation of a File System for Gannet Virtual ...

On the other hand, if large allocation unit is used, the reading and writing file will be
faster than small allocation. Since it needs few numbers of blocks, the overhead time
produced will be less comparing to small allocation blocks. However, this approach
is not space efficient as it has big possibility to waste disk space. Especially, on the
last block as it usually not filled fully by data. Thus, the bigger size of allocation unit,
the bigger disk space to be wasted.
2.2.2 File Allocation
Files are stored on storage media which support direct access. However, there is a
different point of view how to allocate file on a storage media concerning on utility
effectiveness and accessed time aspects. Methods of allocating storage space are
explained as follows.
2.2.2.1 Contiguous Allocation
Contiguous allocation is an allocation mechanism to store data on a contiguous run
of block. This mechanism has significant advantages. Firstly, it is very simple to
implement as it keeps the track of blocks of file by using two attributes: the address
of the first data block on the disk and the number of blocks used. Furthermore, as
this approach only remembers two attributes to manage a file written on the disk,
the overhead is quite low. Second advantage, the read performance is very good as it
only needs single operation which is a seek operation to find the first block of a file.
After that, no seek operation needed as rest of file’s data blocks is stored on a
contiguous run.
10