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

fields will remain the same. However, if a file system designer need to change the
configuration of other fields, for instance number of indirect block in inode
configuration, he should change the code manually.
On storage allocation mechanism, GannetFS inode is dynamically allocated which
allocates block space to a file as needed. As at the creation of an inode, array of data
blocks is not allocated. Those data blocks are being allocated only when there is data
to be stored on data blocks. Hence, file system is efficient in term of space usage.
Direct block represents a list of blocks of a file. In this design, inode can store first 4
blocks addresses on inode itself directly to simplify finding file data. This allows the
first 192 byte of data to be located directly by using simple indexed lookup.
Providing enough direct blocks in the inode to map the data improves the
performance. However, there is a trade-off between the size of the inode and how
much data on inode can map.
However, for file which has size greater than 192 byte, direct block cannot
accommodate the entire file. In order to overcome this constraint, GannetFS inode
can use single indirect block, block_indirect1. When an inode uses an indirect block,
it stores the block address of the indirect block instead of block addresses of the
data. Indirect block contains pointers to the blocks that make up the data of the file.
It does not contain metadata about the file, only pointers to the blocks of the file.
Thus, with one block address, indirect block can access a much larger number of
data blocks.
In GannetFS file system, data block addresses are 4 byte (32 bit). Thus, given a file
system block size of 64 bytes and a block address size of 4 byte, an indirect block
can refer to 16 blocks. Therefore, by using single indirect block, GannetFS can store
file with maximum size 1024 byte.
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