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

The next fields are size and nlink which store information regarding size of file in
byte and number of entries that referenced to the corresponding inode, respectively.
The last three data members are data block management which provides the
connection between the file and the location where those bytes reside on ram
memory. It provides a way to map from a logical file position to a file system block
at some location on memory storage. Block_direct is an array of pointers to the data
blocks in a file. It contain pointer to the actual data block on memory storage.
Therefore, it allows inode to keep track the list of blocks on actual memory that
belong to a particular file. From high level perspective, a file appears as a contiguous
stream of bytes, but on the low level actually the file data is spread over the
memory.
For somewhat larger file, block_indirect1 can be used. This field contains a single
indirect pointer that points to a single indirect block of pointers that point to the
actual data block. For bigger file, singe indirect block, block_indirect1, can be
exhausted. To accommodate this kind of file, double indirect block block_indirect2
should be used as it is a pointer to a block of pointers to pointers to data block. In
other words, it contains two levels of pointer before reaching the actual data block.
Figure 3.3 illustrated how direct and indirect blocks reach the data blocks.
However, first and higher level index approach has an issue regarding the
performance aspect. The higher the index level of data block, the more time
required for accessing data block. Direct block has time performance O(1) as it
contains the actual data blocks. A single indirect block has performance OLog(n) as
it points to an index block which contains the addresses of actual data. Then, the
double indirect block has the worst performance, O (nLogn), since for accessing a
block would require reading in all index blocks to follow the pointer chain until the
target block finally could be reached. However, the performance of single and
double indirect block is not worse than linked list allocation.
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