File Systems | Notion

CSS Optional — Paper-II, Section-A · P-II.III.VII

1. File Concept & Attributes

A file is a named collection of related information recorded on secondary storage. Typical attributes tracked by the OS: name, identifier (unique internal number), type, size, location (pointer to storage blocks), protection (permissions), and timestamps (creation/last-modified/last-accessed).

2. File Access Methods

Method	How it works	Best suited for
Sequential Access	Records read/written strictly in order from the start; reaching record N requires reading through N−1 first	Bulk processing, magnetic tape, log files
Direct (Random) Access	Any block can be accessed directly by its block number, no need to pass through preceding blocks	Databases, indexed files, magnetic/solid-state disks
Indexed Sequential Access (ISAM)	Records kept in sequential order PLUS a separate index maps key values to approximate locations — a fast index lookup narrows the search, then a short sequential scan finds the exact record	Systems needing both fast direct lookup and full sequential scans, e.g. a keyed database table

Random Access (as a memory/storage property, distinct from "Direct Access" file method): any location can be accessed in approximately equal time regardless of physical position — true of RAM and, at the block level, of disks; NOT true of sequential media like magnetic tape.

3. File Organization Techniques

Sequential organisation — simple, efficient for batch processing, slow for locating one record.
Indexed (Indexed-Sequential) organisation — sequential order plus an index for fast lookup.
Direct/Hashed organisation — a hash function maps a key directly to a storage location, giving near-O(1) average access without an index structure.

4. Logical File System — Functions

The Logical File System layer manages metadata and provides the file abstraction to applications (as opposed to the physical layer that manages actual disk blocks):

Maintains the directory structure (hierarchy, naming).
Manages file metadata (owner, permissions, size, timestamps, location pointers) typically in a File Control Block (FCB) / inode.
Translates symbolic file names into the identifiers lower layers use.
Enforces access control — checks permissions before allowing an operation.
Presents files as a simple byte/record stream, hiding physical block-allocation details.

5. UNIX vs Windows File Systems (a favourite comparative question)

Aspect	UNIX (e.g. ext4)	Windows (NTFS)
Structure	Single hierarchical tree rooted at "/"; all devices mounted as subdirectories of one tree	Multiple independent root volumes, each with its own drive letter (C:, D:, …)
Permissions	Simple owner/group/other model, read-write-execute (rwx) bits	Access Control Lists (ACLs) — fine-grained, per-user/per-group permissions
Metadata structure	<b>Inodes</b> — data structures holding metadata + block pointers	<b>Master File Table (MFT)</b> tracking metadata and locations
Naming	Case-sensitive	Case-insensitive but case-preserving
Philosophy	"Everything is a file" — devices/pipes treated as files too	Richer per-object security model for desktop/enterprise use

Both support journaling for crash recovery. UNIX's simpler, uniform model suits portable, scriptable, multi-user server environments; NTFS's richer ACL-based model suits Windows' typical desktop/enterprise environments needing granular per-user security control.