Storage Hardware#

Storage on hardware for DBMS differs from CPU architecture by:

  • No pointer dereferences, API is used instead:

    • READ: Transfers a page of data from disk to RAM

    • WRITE: Transfers a page of data from RAM to disk

  • Very slow API calls

  • Minimizes pointer errors

Magnetic Disk (HDD)#

Still in use today because how cheap it is.

Platter : The assortment of disks in an HDD

Arm Assembly : Move in or out the head to a track

Sector : A fixed amount of data taking the space of a arc sector of a disk.

Accessing Disk#

Seek time : Time it takes to move disk head on track * 2-3 ms

Rotational delay : Waiting for block to rotate under head to the right place * 0-4 ms

Transfer time : Moving data from disk surface * ~0.25 ms per 64KB page

Flash Memory (SSD)#

Let’s focus on NAND technology,

* Fine grain reads (4-8k reads), course-grain write (1-2 MB writes)

  • **Pros: **

    • SSD is fast read and predictable with writes being slower (i.e., random 4K writes).

  • **Cons: **

    • Only 2k-3k erasures (i.e., multiple writes), the cell will fail.

    • Slows down from

      • Wear leveling (preventing a cell from multiple writes)

      • Write amplification (movement of data from cell to cell)

      • Writing a single unit may require multiple units in order to move units around

      • Large transfer sizes