Operating Systems - Chapter 8

File system (FS)

The part of the OS that implements the concept of files.

File

A named collection of information managed on secondary storage by the FS; may be an unstructured byte stream or a series of records.

Record

A structure of related data items, identified within a file by a record number or unique key field.

Access method

A set of operations for accessing files; common types are sequential (next n bytes/next record) and direct (access by record number or key).

File directory (folder)

A special-purpose file that records information about other files and possibly other directories.

Tree-structured directory hierarchy

A directory structure where every file/directory (except the root) has exactly one parent.

Root

The highest-level directory in a tree hierarchy; has no parent.

Absolute path name

The full path from the root to a file, concatenating directory/file names.

Current working directory

A user-designated directory used as the starting point for relative paths.

Relative path name

A path starting from the current working directory.

Directed acyclic directory hierarchy

A structure allowing a file/directory to have more than one parent (no cycles).

Link

An operation/entry creating an additional path to a file or directory ("link path1 path2").

Reference count

A non-negative integer tracking how many directories point to a file; the file is deleted only when the count reaches 0 after decrementing (i.e., last link removed).

Symbolic link (shortcut)

A directory entry pointing to a file/directory, but whose deletion removes only the link, not the file itself; allows sharing without cycles or multiple "true" parents.

Garbage collection

An algorithm needed to reclaim unreachable files/directories that can result from cycles in a general graph structure.

File attributes

Metadata such as size, type, location, protection, and use (creation/access/modification times).

File control block (FCB)

A data structure (stored apart from the directory) containing all attributes of a file; called an i-node in Unix.

Fixed-length entries

Simple, fast, but limits file name length.

Variable-length entries (with length field)

Allows variable names but requires coalescing holes on deletion.

Variable-length entries with two pointers (K, L)

Entry length (K) and name-end (L) pointers; makes renaming easier.

Create (file operation)

Allocates and initializes a new FCB and directory entry for a new file.

Destroy (file operation)

Frees a file's FCB, directory entry, and disk blocks.

Open file table (OFT)

An in-memory data structure tracking all currently open files for efficient access.

Open (file operation)

Verifies access rights, allocates an OFT entry, copies FCB info into it, and returns an OFT index.

Read (file operation)

Copies data from an open file's r/w buffer into memory ("read i, m, n").

r/w buffer

A buffer holding the current disk block of an open file to avoid repeated disk access.

Write (file operation)

Copies data from memory into an open file's r/w buffer, allocating new disk blocks as needed ("write i, m, n").

Seek (file operation)

Moves the current position of an open file to a specified position k, loading a new block into the r/w buffer if needed ("seek i, k").

Close (file operation)

Saves the current file state (length, modified buffer) back to the FCB/disk and frees the OFT entry.

Disk block

A fixed-size sequence of bytes on disk, accessed as a single unit.

Contiguous allocation

Files stored as a contiguous sequence of disk blocks; fast access but causes fragmentation and expansion difficulty.

Linked allocation

Blocks scattered on disk, each pointing to the next; easy to expand but slow sequential/no direct access, less reliable.

Clustered allocation

Sequences of contiguous blocks ("clusters") linked together; a compromise between contiguous and linked.

File allocation table (FAT)

An array (often cached in memory) tracking block chains for files, separate from the blocks themselves; supports efficient sequential and direct access.

Indexed allocation

A dedicated index table per file lists all its block numbers; supports fast sequential and direct access but requires deciding table size upfront (or using expanding multi-level indices, as in Unix).

Bitmap

A data structure with one bit per disk block (1 = allocated, 0 = free) used for free space management.