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File Allocation Table (FAT)
The OS has the task of managing and creating file systems on the HDD.
A commonly used file system is the FAT.
In a HDD there are clusters where the data is stored, there is
a minimum amount of data that can be stored in a
cluster and any amount of data that is less than
minimum wastes the remaining memory. A typical
cluster size can range from 32, 4kb or 2kb in size.
Each of these clusters is numbered so the FAT
can locate it.
Question Section 1
What are the major differences between FAT and NTFS
Systems?
NTFS or New Technology File System is a new file management structure that can
about on consumer machines around the introduction of Windows XP in 2001.
Compatibility
FAT – is read/write compatible with the majority of current and obsolete operating
systems.
NTFS – Only read/write compatible with windows and mac operating systems that
come after its introduction in 2001.
Storage Sizes
FAT – only supports up to 4GB files and its volume tops out at 2TB in size.
NTFS – supports up to 16EB (exabytes really bloody big) meaning that
uncompressed videos and data can be stored.
Back to FAT stuff
Files are stored in one or more clusters, these clusters are unlikely to be next to
each other because of the way files are allocated space on the HDD. Because they
are scattered they form a ‘chain’ with all their separate clusters. The FAT system
keeps at track of these chains.
The FAT is a data table stored on the Hard Disk. It contains a number of records,
each record describes details about each cluster. So the FAT keeps a record of
every cluster on the
HDD so that chains
can be traversed.
The FAT does not however store the location of the first cluster of the file. Another
table called the Directory Table stores that data.
Each entry into this table provides details of every file within that directory (folder).
The Details include:
The file manager will look
at the directory table for
the starting cluster, then to
the FAT to find the other
pieces of data in the chain.
Question Section 2
What does VFAT mean and give a brief description.
VFAT (Virtual File Allocation Table) is a part of the operating system that handles
long file names that cannot be handled by the FAT programming.
Saving Files
When a file is saved it will traverse the FAT looking for a free cluster. Once one is
found it notes that starting cluster in the Directory table.
It then writes a chunk of the file to that cluster.
Next it looks for the next free cluster FAT and writes it to the next cluster until all of
the data is written. If the last chunk of data is smaller than a cluster then that unused
space is wasted.
The size of the cluster will decide how efficient the filing system is. If the cluster is
small like 2kb nearly all of the cluster will be used guaranteed but this will leave less
cluster on the drive so less memory available overall.
Deleting Files
When a file is deleted the FAT starts with the first cluster and marks that cluster as
free to write onto, it then continues along the chain and does the same until all of it
is flagged as clear for writing onto. At the end it goes to the directory table and flags
the file as deleted at the end of this process.
The next file to be saved may come across those freed up clusters and then simply
write that data over the data that is there, corrupting the old data.
File recovery applications find these clusters marked as deleted and either flag them
as not deleted allowing the data to be accessed again or if it has been written over,
take the remaining readable data.
Fragmentation
Because of the way the FAT works data can be saved in several locations all over
the hard disk. This means that when the data is to be accessed again the needle
has to move all around the platter in order to get to the chain of clusters. This can
both lead the hard drive to fail sooner a it works harder to read files, and it also
slows the boot times of the data that is stored.
In order to prevent this a utility called a disk defragmenter goes through all of the
files on the system and reorganises them so that the files are stored next to each
other in terms of clusters so that they are loaded with less work from the needle.