Mass storage #

Magnetic disks #

• we want to keep the same rate of data moving under the head.

• a track is 2D but the entire disk is 3D

Watch: Inside of a hard disk drive

Disk performance #

• seek and rotate times dominate the total latency
• hard disks are at the milisecond level

Disk history #

Solid state disks #

• SSDs are generally faster because they do not have any moving parts, thus no seek and rotate times, this greatly speeds up the total latency
• SSDs are at the microsecond level

Magnetic tape #

• magnetic tape remains in use today as a tertiary storage
• ideal for backup/logs

Disks vs memory #

Disk scheduling #

The one component of disk latency that we have the ability to optimize is the seek time. If we minimize the seek distance we can minimize the seek time.

• needs to know the entire sequence in advance

• remember that the “giant leap” here is still a physical movement

Selecting a disk scheduling algorithm #

RAID #