Operating System File Management and Memory Management

A comprehensive study set focused on the principles and mechanisms related to file and memory management in operating systems.

File as OS Abstraction

A container for data, managed by the OS, that is persistent and can be named.

Non-volatile

Type of memory or storage where data survives after termination.

File Metadata

Information stored in the FCB/inode about a file including size, ownership, permissions, and access type.

ctime

The time when the metadata of a file was last changed.

mtime

The time when the contents of a file were last modified.

Atime

The time when a file was last accessed.

Ownership

Refers to the user and group that have permission to manage a file.

Permissions

Rules that determine who can read, write, or execute a file.

Sequential Access

Accessing data in order from the current position, moving step by step through the file.

Random Access

Accessing data at any location by specifying an offset, bypassing sequential order.

Indexed Access

Accessing data using a key or index structure rather than byte positions.

Open-file properties

Properties of a file related to its current position and access mode.

Current Position (CP)

Indicator of where the next read or write will occur in a file.

File Name

A label used to identify a file, stored in the directory and not a property of the file itself.

File Extension

A convention to manage file associations, typically appearing as part of a file's name.

File Control Block (FCB)

A data structure that contains all properties of a file and its location on disk.

Open File Descriptor

A unique identifier returned by the OS when a file is opened.

Create File Operation

Allocates an entry in the file system and creates a new file.

Delete File Operation

Removes a file's directory entry, marking it for deletion in the file system.

Open File Operation

Prepares a file for use by returning a file descriptor.

Close File Operation

Releases file resources and may delete the file if unlinked.

Write Operation

Copies data from memory to a file and updates the current position.

Read Operation

Copies data from a file to memory and updates the current position.

Seek Operation

Changes the current file position without reading or writing data.

File Locking

Mechanism that restricts access to a file to prevent conflicts.

Sync/Flush

Forces buffered data to be written to secondary storage.

Master Boot Record (MBR)

Data structure that contains information about all disk partitions.

Contiguous Allocation

All blocks are allocated on the same track in adjacent sectors.

Linked-list Allocation

Each block contains a pointer to the next, allowing flexible memory usage.

File Allocation Table (FAT)

Maintains a chain of blocks in a global table for access.

Indexed Allocation

Directly stores all block numbers for fast access through the FCB.

Free Space Management

Tracking unused disk blocks to allocate them to files when needed.

Dirty State

A condition indicating that the file system may be inconsistent due to incomplete operations.

Journaling

Maintaining a log of intended changes to allow recovery after failures.

Virtual Memory

Uses secondary storage as if it were RAM, expanding usable memory space.

Page Table

Data structure that maps virtual addresses to physical addresses.

Page Fault

Occurs when a program accesses a page not currently in RAM.

Demand Paging

Loading pages into memory only when they are needed.

FIFO Replacement Policy

Evicts the oldest page in memory first among all pages.

LRU Replacement Policy

Evicts the least recently used page in memory.

Clock Replacement Policy

A practical approximation of LRU, using accessed bits to track usage.

Bélády's Anomaly

Observation where increasing frames can lead to more page faults under FIFO.

Fixed Allocation

Assigns a fixed number of frames to each process up front.

Dynamic Allocation

Processes draw from a common pool of frames for better flexibility.

Thrashing

Condition where excessive page faults hinder program execution.

Working-set window (Δ)

Counts distinct pages accessed in a recent time frame to allocate frames.

Page-fault rate (PFR) Algorithm

Regulates frame allocation based on observed fault rates.

File Name Mapping

Directory structure mapping file names to their respective FCBs.

Soft Link

Directory entry that stores a path to another file.

Hard Link

Directory entry that points directly to the same FCB as the original file.

Swap Partition

A fixed-sized area on disk dedicated to swapping data in/out of RAM.

Swap File

A regular file that temporarily holds pages from memory.

Memory-Mapped Files

Files directly mapped into a process's virtual address space.

Eviction Policy

Strategy used by an OS to decide which page to remove from memory.

Accessed Bit (A)

Indicates whether a page has been used recently, influencing replacement choices.

Dirty Bit (D)

Indicates whether a page has been modified and needs saving upon eviction.

Write/Protection Bit (W)

Determines permissions for reading and writing to a memory page.

L Bit (Locked Bit)

Indicates a page cannot be evicted from memory.

Sectors

Smallest storage unit on a hard disk, typically 4 KB.

Tracks

Concentric circles on a hard disk where sectors are located.

File System

An organization method for storing and retrieving files on a storage device.

Access Mode

Specifies how a file can be opened: for reading, writing, or appending.

Buffered Data

Data stored temporarily before being written to a file or disk.

Recovery Process

Restoring consistency in file systems after a crash or failure.

Metadata Mismatches

A condition where file metadata does not align with actual file contents.

File System Structures

Organizational frameworks such as directories, tables, and FCBs used in file management.

Path Component

Individual segments in a file's directory path used for locating files.

Cache Copy

A temporary copy of file metadata stored in RAM after opening a file.

File System Cleaning

Process of restoring the file system to a consistent state.

Bit Vector

A bitmap representation of free and used blocks within a storage device.

File Linking

The process of allowing a single file to be referenced by multiple names.

File Creation Behavior

Determines the outcome when trying to create an existing file.

Accessing Data via Keys

Utilizing metadata keys instead of byte positions to access file data.

File Recovery Techniques

Methods used to restore lost or damaged file data post-failure.

Dynamic Memory Management

Processes that handle memory allocation and deallocation during runtime.

Operating System Abstractions

Higher-level representations of hardware resources managed by the OS.

Secondary Storage

Permanent storage solution for files that remains intact when the system is powered down.

RAM-based Files

Temporary files stored in RAM for speed, often used for caching.

User Permissions

Configurations determining what actions a user can perform on a file.

File System Initialization

Setup process wherein file management structures and controls are established.

Examining File Data

Process of accessing and manipulating the content of files.

OS File Operations

Fundamental methods provided by an operating system for managing files.

Data Integrity

Ensuring that data is accurate, consistent and unrecoverable after it is created.

System Recovery Mechanism

Systems in place to recover from failures and restore normal functionality.