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Unit 3: File System Hierarchy

processes executing on behalf of different users can access the information in files. File protection notes
is a fundamental property of files because it allows different people to store their information on
a shared computer, with the confidence that the information can be kept confidential.
In addition to these functions, the file manager also provides a logical way for users to organize
files in secondary storage.
In this unit we take a brief look at how the operating system deals with files system. We will
discuss how an operating system keeps track of files. The hard disk is comprised of a large number
of sequentially numbered sectors. As files are created, free sectors are allocated to hold the file
contents and marked as allocated. To keep track of the sectors and whether they are allocated
or free, and to which file they belong, the operating system maintains a number of tables. In this
unit we will learn the how the disk space is allocated and how the efficiency and performance of
file system can be increased. Hard drives are not perfect: they develop defects due to magnetic
dropout and imperfect manufacturing. On more primitive disks, this is checked when the disk is
formatted and these damaged sectors are avoided. If sector becomes damaged under operation,
the structure of the disk must be patched up by some repair program. Usually the data are lost.
In this unit data recovery is also discussed.

3.1 file systems

A file system is a method for storing and organizing computer files and the data they contain to
make it easy to find and access them. File systems may use a data storage device such as a hard
disk or CD-ROM and involve maintaining the physical location of the files, they might provide
access to data on a file server by acting as clients for a network protocol (e.g., NFS, SMB, or 9P
clients), or they may be virtual and exist only as an access method for virtual data.
More formally, a file system is a set of abstract data types that are implemented for the storage,
hierarchical organization, manipulation, navigation, access, and retrieval of data. File systems
share much in common with database technology, but it is debatable whether a file system can
be classified as a special-purpose database (DBMS).

3.1.1 types of file systems

File system types can be classified into disk file systems, network file systems and special purpose
file systems.
1. Disk file systems: A disk file system is a file system designed for the storage of files on a
data storage device, most commonly a disk drive, which might be directly or indirectly
connected to the computer.

Example: Disk file systems include FAT, FAT32, NTFS, HFS and HFS+, ext2, ext3,
ISO 9660, ODS-5, and UDF. Some disk file systems are journaling file systems or versioning
file systems.
2. Flash file systems: A flash file system is a file system designed for storing files on flash
memory devices. These are becoming more prevalent as the number of mobile devices is
increasing, and the capacity of flash memories catches up with hard drives.

While a block device layer can emulate a disk drive so that a disk file system can be used
on a flash device, this is suboptimal for several reasons:
Erasing blocks: Flash memory blocks have to be explicitly erased before they can be written
to.
Random access: Disk file systems are optimized to avoid disk seeks whenever possible, due
to the high cost of seeking. Flash memory devices impose no seek latency.

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