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Storage System

The storage system manages non-volatile data storage across various physical mediums.

Hard Disk Drive (HDD) Structure​

A traditional HDD consists of one or more rotating platters with magnetic surfaces.

  • Track: Concentric circles on the platter.
  • Sector: A small portion of a track (usually 512 bytes or 4 KB).
  • Cylinder: A set of tracks across multiple platters at the same arm position.
  • Seek Time: The time to move the disk arm to the correct track.
  • Rotational Latency: The time it takes for the desired sector to rotate under the head.

Solid-State Drive (SSD)​

SSDs use NAND flash memory with no moving parts. They are much faster but have unique characteristics:

  • Pages and Blocks: Data is read and written in pages (e.g., 8-16 KB) but must be erased in blocks (e.g., 128-256 pages).
  • FTL (Flash Translation Layer): A mapping between logical block addresses (LBAs) and physical flash locations. It handles Wear Leveling (distributing writes to avoid premature failure) and Garbage Collection.

RAID (Redundant Array of Independent Disks)​

RAID combines multiple physical disks into a single logical unit to improve performance and/or reliability.

  • RAID 0 (Striping): Splits data across disks. Fast, but no redundancy (if one disk fails, all data is lost).
  • RAID 1 (Mirroring): Duplicates data across disks. High reliability, but half capacity.
  • RAID 5 (Parity): Stripes data and parity across at least three disks. Provides redundancy with good performance.
  • RAID 6 (Double Parity): Can survive two disk failures.
  • RAID 10 (1+0): A stripe of mirrored disks. Combines high performance and high reliability.

Disk Scheduling Algorithms​

The kernel must decide the order in which to fulfill pending disk I/O requests to minimize seek time.

SCAN (Elevator Algorithm)​

The disk arm starts at one end and moves toward the other, fulfilling requests along the way, then reverses direction.

  • Pros: Fairer than FCFS; reduces starvation.

C-SCAN (Circular SCAN)​

Similar to SCAN, but when it reaches the end, it immediately returns to the beginning without fulfilling requests on the way back.

  • Pros: Provides a more uniform wait time.