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FileSystems Home
File
systems organize the data stored on computer hard drives, keeping track of the
physical locations of all data elements on disk while allowing users to quickly
and reliably retrieve files when needed.
The file system acts as a digital index that lets a computer instantly find a specific file, regardless of the size or configuration of the storage drive or where the data bytes associated with the file sit on the drive's storage platters.
Every operating
system, from MS-DOS to Windows 95, Windows XP and Linux, has its own file
system. But although all file systems perform the same basic functions, they
vary in design and sophistication.
FAT - File systems have
come a long way since MS-DOS and early versions of Windows. Those operating
systems organized files under the FAT file system, which represents logical
areas of the disk in allocation units called clusters, and maps the locations
of file data to those areas using a file allocation table (FAT). FAT is also
called FAT16 because it uses a 16-bit address space for tracking files and
clusters.
FAT clusters
vary with the size of the disk. FAT's 16-bit address space can support up to
65,536 clusters (216). With a 65MB disk, cluster sizes were just 1KB in size,
but they ballooned as disks emerged that were able to hold gigabytes of data.
And since only a single file can be written to a cluster, this created
inefficiencies that ended up wasting as much as 50% of available space on a 2GB
disk drive.
FAT32 - FAT32, which debuted
with Windows 95 OEM Service Release 2 (OSR2), introduced a 32-bit address
space. By increasing the size of the file allocation table, it could support
more clusters that were smaller in size on large disk drives, reducing the
potential for wasted drive space.
Another FAT32
innovation was that it could handle file names with up to 255 characters,
whereas FAT could only handle names with up to eight characters. Users could
finally create long file names to better describe the contents.
The advent of
FAT32 extended the maximum addressable volume size from 2GB to 2TB and improved
reliability by allowing the system to switch to a copy of the file allocation
table if the default copy should become damaged. But FAT32 also added to file
system overhead and was therefore inefficient to run on disks smaller than
260MB.
NTFS - The next development
in Windows file systems was the New Technology File System (NTFS), introduced
with Windows NT (which also supported FAT32). With a 64-bit address space and
the ability to vary cluster size independently of the disk drive size, NTFS
virtually eliminated the cluster size limitation problem.
It also brought
other benefits, including file and directory security attributes, file
encryption and support for storage volumes of up to 16TB and 232 clusters.
NTFS replaced
the familiar file allocation table format with the Master File Table (MFT),
which holds more information about files than did FAT. The MFT references all
files and directories on the disk drive, including associated metadata such as
security settings.
The NTFS also
introduced a high level of fault tolerance. It logs disk operation activity
prior to committing the transaction. If the system crashes during an update, it
can examine the log file and restore the data. When read or write errors occur
during normal operation, NTFS automatically identifies and blocks out the bad
clusters and copies the data to a new location. Finally, NTFS creates a mirror
of the MFT and can revert to the mirror should the original fail.
NTFS's overhead
makes it unsuitable for disks smaller than 400MB, and it can't be used on
floppy disks. Instead, Windows must write to formatted diskettes using FAT32.
Ext2 & Ext3 - The Linux file system, called
Extended File System 2, evolved to rectify limitations of Linux's original file
system, Ext, which the operating system inherited from its Minix predecessor.
Under the Minix file system, the maximum file system size was restricted to
64MB and file names to 14 characters.
Ext supported
2GB file systems and 255-character file names but suffered from some
performance limitations. Ext2 supports 4TB file systems and 255 character file
names and remedies those problems.
The Ext2
architecture uses a data structure called identification nodes (inodes) to
refer to and locate files and associated data. The inode table includes the
file type, size, access rights, pointers to associated data blocks and other
attributes. The file system organizes disk space into groups of blocks, which
contain both inode information and associated data blocks.
The Linux
kernel uses the Virtual File System layer, which interacts with the file system
to perform disk I/O. This gives Linux the ability to support multiple file
systems, including DOS, FAT16 and FAT32 (which it supports as a native file
system).
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FILE SYSTEM SUPPORT BY OPERATING SYSTEM |
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