Disk Structure - aryanjoshi0823/5143-Operating-System GitHub Wiki

Disk Structure in Operating Systems

The disk structure in an operating system refers to how data is physically and logically organized and managed on a storage device, such as a hard disk drive (HDD) or solid-state drive (SSD). The disk structure ensures efficient storage, retrieval, and management of data.

Figure - Moving-head disk mechanism.

1. Physical Structure of a Disk

A hard disk or similar storage device has a specific physical layout:

Components

  • Platters: Circular disks made of metal or glass, coated with a magnetic material to store data.
  • Tracks: Concentric circular paths on the platter's surface where data is recorded.
  • Sectors: Small subdivisions of tracks. A sector is the smallest physical storage unit, typically 512 bytes or 4 KB.
  • Cylinders: A vertical stack of tracks across all platters at the same radius.
  • Read/Write Head: A mechanical arm that reads or writes data on the platter surface.
  • Spindle: A motorized axis that spins the platters.

Data Addressing

Data on the disk is addressed using a CHS (Cylinder-Head-Sector) format or the more modern LBA (Logical Block Addressing).

2. Logical Structure of a Disk

The logical structure of a disk abstracts the physical layout into a usable format for the operating system. It includes:

Partitions

  • A disk can be divided into partitions, each appearing as a separate logical drive.
  • Common types:
    • Primary Partition: Bootable and contains an OS or file system.
    • Extended Partition: Can hold multiple logical drives.
    • Logical Partition: A section within an extended partition.

File Systems

  • The file system organizes and manages data within partitions. Examples include:
    • NTFS (Windows), ext4 (Linux), HFS+ (Mac).
  • Responsibilities:
    • Managing files and directories.
    • Tracking free and occupied disk space.
    • Handling permissions and metadata.

Blocks

  • Data is stored in fixed-sized blocks, typically 4 KB. Blocks are the smallest logical unit of storage.

3. Disk Management and Structures

The operating system relies on specific structures for managing the disk:

Master Boot Record (MBR):

  • Located in the first sector of the disk.
  • Contains:
    • Partition table.
    • Boot loader to start the operating system.

GUID Partition Table (GPT):

  • A modern replacement for MBR, supporting larger disks and more partitions.

Disk Scheduling:

  • Algorithms used by the OS to manage read/write requests efficiently:
    • FCFS (First Come, First Served)
    • SSTF (Shortest Seek Time First)
    • SCAN (Elevator Algorithm)
    • C-SCAN (Circular SCAN)

Inodes:

  • Used in file systems like ext4 to store file metadata (e.g., size, location, permissions).

4. Disk Access Time

Disk performance is characterized by:

  1. Seek Time: Time taken by the read/write head to move to the desired track.
  2. Rotational Latency: Time for the platter to rotate to the desired sector.
  3. Transfer Time: Time to transfer data from the disk to memory.

5. RAID (Redundant Array of Independent Disks)

RAID combines multiple disks for performance and reliability:

  • RAID 0: Striping (Performance)
  • RAID 1: Mirroring (Reliability)
  • RAID 5: Striping with Parity (Balanced)

6. Disk Caching

  • A portion of memory is used to temporarily store frequently accessed data, reducing access times.