Magnetic (hard) disk

• Magnetic disk


  Hard disk
• Floppy disks


  Also magnetic, commonly used in the 1980s
• HDD


  Hard disk drive, a mechanical device with many moving parts
• HDDs are sold as sealed units, so the disk medium and drive mechanism are well protected from contamination and wear and tear
• Data representation on disk surface


  Magnetised dots, binary data represented through 'magnetised' and 'not-magnetised' states
• Read-write process


  1. Disk spins at high speed

  2. Faster spin means quicker data retrieval
• Disk spin speeds


  • 5,400 rpm
  • 7,200 rpm
  • 15,000 rpm (higher-specification drives)
• Standard HDD read-write access speeds

  80 to 200MB/s
• Variation in access speeds is determined by disk configuration, file size, and disk fragmentation
• Fragmentation occurs when there is insufficient contiguous space to store the file
• If a disk is nearly full, it will take longer to find space to write file segments, and it will take longer to access a fragmented file
• Magnetic hard disks


  • High capacity
  • Low unit cost compared to SSDs
• Magnetic hard disks are best suited to desktop and laptop devices where low purchase cost is the most important selection criterion
• Disk unit


  • Comprises one or more metal platters with a magnetic coating
  • Each platter is double-sided
  • Number of platters determines total storage capacity
• Binary data


  Stored on the platters as a series of polarised states, each representing 1 or 0
• Disk structure


  1. Platters broken down into concentric tracks

  2. Tracks further broken down into sectors

  3. Contiguous group of sectors called a cluster (also called a block or allocation unit)

  4. Cluster is the smallest unit of storage that can be addressed by the file system

  5. Cluster size varies depending on disk size and formatting
• The location of each file is recorded in a file allocation record. This allows the file to be retrieved, and free space to be identified for new files to be stored. When the disk is fairly full, it is common for files to be split across many non-contiguous blocks. This requires the file allocation record to keep track of not only where the file segments are, but also the order in which they should be reassembled. If this record is lost or corrupted, the file data will still be present on the disk, but it will be impossible to access it in a meaningful way, unless the record can be rebuilt.
• Device


  • Has two mechanical arms, one for each side of each platter
  • At the end of the arm is a read-write head with a tiny magnet
  • Head does not touch the platter, there is a very thin gap between the head and the disk surface
• Platters


  Spin at a speed of 7,200 rpm (revolutions per minute)
• Reading and writing data

  1. Head sweeps across the tracks

  2. Sectors pass underneath the head as the disk rotates
• This allows any block to be accessed directly, without having to read through the entire contents of the disk