Solid state disk

• SSD


  Solid-state disk, solid-state drive, or solid-state device
• SSD


  • Electronic device that uses flash memory
  • Has no moving parts
• Compared to magnetic hard disks

  SSDs offer many advantages
• Main advantage of SSDs

  Speed of access
• Read-write access speeds for a standard SSD are in the range of 200–500 MB/s
• SSDs will always outperform magnetic or optical drives
• SSDs


  • Absence of moving parts means mechanical failure is far less likely
  • Reduced likelihood of damage through day-to-day bumps and knocks make an SSD a better choice for portable devices
  • Lack of moving parts reduces the amount of energy consumed and heat generated
• SSDs have a limited lifespan
• Limitations of SSDs


  • Number of times an SSD can be written to is finite
  • Memory cell 'leakage' can be a problem if devices are not powered up from time to time
  • Not suitable for long-term archiving of data
• At the time of writing (2020), SSD storage is still more expensive per GB than magnetic storage
• High-end computer systems will sometimes have large-capacity solid-state disks, but this will add significantly to the overall cost of the device
• SSDs are usually built into a physical enclosure that mirrors those used for magnetic disk drives
• Suitable uses for SSDs


  • Storing files that are read or changed frequently
  • Storing operating system and frequently accessed applications and data
• It is becoming increasingly common for computer systems to have both magnetic and solid-state disks
• Advantages of having both magnetic and solid-state disks

  • Small SSD can be used to store the operating system and frequently accessed files/applications
  • Less frequently accessed files/applications can be stored on the larger magnetic disk
• Solid-state disks are made up of a controller and a bank of millions of NAND 'flash' memory cells.
• Each cell has a floating gate transistor that allows an electrical charge to be trapped. A charged cell represents a 0, and a cell with no trapped charge represents a 1.
• NAND flash cells are organised in terms of pages and blocks. A page is a cell within a grid, and each row of cells is a block.
• Data is read from an SSD by page. It can also be written a page at a time, as long as the surrounding cells are empty. If a file is to be updated, this cannot be done in situ. The relevant block containing the pages must be copied to main memory and updated. It will then be written to a new area of the disk and the original block(s) will be erased.
• SSDs become slower as they fill up. A new empty drive will be full of pages that can be written to immediately. When the disk is nearly full, writing and updating will become relatively slow operations as a result of having to copy out, then rewrite existing pages within a block.