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

partition for directory structure to 32 byte and file names to 24 characters. The use
of fixed-length of directory entries, in this case 32 bytes, is a trade off involving
speed and storage. GannetFS directory is very simple and space efficient for looking
up and storing entries. However, there is wasted on storage, if the name of file is less
than 20 characters.
It is different with directory entry on other UNIX which breaks data blocks into
variable-length directory entry in order to allow name of files to be of nearly
arbitrary length. Thus, the size of directory entry structure varies depending on the
filename length. Therefore, there is no space wasted for small file name. However, as
an entry is deleted, the entry’s space from deleted entry cannot be reused for new
entries if the length of file name longer than the deleted one. Hence, the size of
directory tends to grow despite there are number of entries deleted.
However, the usage of fixed length on GannetFS overcomes that problem. Since the
size is fixed, the entry’s space from deleted entry can be used for new entry.
Moreover, to minimize the wasted space, the maximum limit for entry’s file name is
designed to accommodate the mean file name length. Therefore, this approach
provides more space efficiency.
3.2.2.3 Translator Data Structure Design
A common request on file system is to look up a file name in a directory. This
request commonly stated using name path. However, the system cannot understand
the path name as system accessing files and directories by accessing their inodes.
Moreover, GannetFS does not support relative address. As the user of GannetFS is
service cores on Gannet system which is not human, there is no need to establish a
relative address. Hence, a mechanism to resolve an absolute path name into an
inode is required. GannetFS uses translator structure to resolve the path name
required.
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