Abstract
The File System provides an abstraction layer mapping human-readable, byte-oriented logical structures (Files and Directories) onto non-volatile physical storage media. It manages file naming, access, storage allocation, user permissions, and crash durability, hiding physical hardware complexities behind a standardized interface (VFS in Unix, IFS in Windows).
File System Components & Core Abstractions
An OS file system manages five fundamental components:
flowchart TD NAMING["<b>1. Naming</b><br/>Referencing data via files and directories."] ACCESS["<b>2. File Access</b><br/>Read, write, and control operations."] MGMT["<b>3. Disk Management</b><br/>Block allocation, arranging data, mapping data to blocks."] PROT["<b>4. Protection & Permissions</b><br/>Protecting data across different users."] RELIAB["<b>5. Reliability & Durability</b><br/>Preserving data across system crashes."]
Key Abstractions
- Files: A named collection of bytes stored on durable media.
- Properties: Size, owner, last modified time, permissions.
- Types: Understood by filesystem (
link,character,block) vs. OS/runtime (text,source,object,executable, untyped). Encoded via name or content.
- Directories: A logical organization mechanism consisting of a list of entries mapping .
- Directory lists are typically unordered on disk and sorted by user utilities.
- Unix Philosophy: “Everything is a file” — directories are stored internally as files.
- Virtual File System Interface: OSs abstract diverse file systems behind a unified API (VFS on Unix, IFS on Windows).
Physical Reality vs. File System Abstraction
The file system translates raw physical disk characteristics into protected, reliable software abstractions:
| Physical Hardware Reality | File System Abstraction |
|---|---|
| Block-Oriented (Logical Block Address) | Byte-Oriented stream |
| Unnamed block indexes | Named Files |
| No protection among users | Users protected from each other |
| Data vulnerable to crash corruption | Robust and durable across machine failures |
File Access Patterns & Sharing
Access Patterns
- Sequential Access: Bytes are read strictly in order from start to finish.
- Random Access: Address any arbitrary byte offset directly without reading prior bytes (e.g., databases, swap files).
- Indexed Access: Uses index structures (e.g., hash tables, dictionaries) to look up specific block contents.
File Sharing & Concurrency
File sharing provides the foundation for communication and synchronization. Key issues include:
- Semantics when one process reads while another writes.
- Semantics when two processes open a file for writing simultaneously.
- Coordination primitives (e.g., file locking).
Protection & Access Rights
Protection systems verify whether an action performed by a subject on an object is allowed.
graph LR Subject["Subject (User / Process)"] --> Action["Action (r / w / x)"] --> Object["Object (File / Directory)"]
Unix Access Control Model
Unix divides access rights into three user classes:
- Owner (
u): User who owns the entry. - Group (
g): User group associated with the entry. - Public / Others (
o): All other system users.
Permissions for each class specify Read (r), Write (w), and Execute (x) access.
chmod: Modifies access permissions.chown: Changes file ownership.
Root and Administrative Privileges
root(Unix) andAdministrator(Windows) bypass all kernel protection checks.- Best Practice: Always operate under a standard user account and utilize privilege escalation (
sudo) only when administrative modifications are required.
Memory & Storage Technology Spectrum
Storage and memory technologies exhibit distinct latency, throughput, capacity, and cost trade-offs across the hardware hierarchy:
| Metric | DRAM | Non-Volatile Memory (NVM) | Solid State Disks (SSD) | Hard Disk Drives (HDD) |
|---|---|---|---|---|
| Volatility | Volatile | Non-Volatile | Non-Volatile | Non-Volatile |
| Latency | ||||
| Bandwidth | A few | A few | ||
| Capacity | Tens of GB / module | Tens to hundreds of GB / module | ||
| Cost | Highest (\$$$$$$) | High (\$$$$) | Moderate (\$$$) | Lowest (\$$) |