This chapter discusses the hard disk, basically.

Overview of Mass Storage

Disks are attached to the computer via I/O bus.

Magnetic disks provide bulk secondary storage.

• Tranfer Rate: Rate of data flow between drive and computer.
• Positioning Time: Seek Time + Rotational Latency
  • Seek Time: Time to move disk arm to desired cylinder.
  • Rotational Latency: Time for desired sector to rotate under disk head.
• Head Crash: When disk head makes contact with disk surface.

Solid state disks are smaller, cost more, are faster.

Magnetic tapes are relatively permanent and hold large quantities of data.

• Random access is way slower, but way cheaper; so good for backup and storage/transfer.

Disk Structure

Disk drives are addressed as large 1D arrays of logical blocks.

• Logical Block: Smallest unit of transfer (usually 512b)

TODO Diagram and explanation of how the logical blocks are mapped to the sectors of the disk.

In theory, this lets us convert logical block numbers into old-school disk addresses (cylinder number, track number, sector number), but this is difficult due to:

• Bad Sectors
• Number of sectors per track isn’t consntant on some drives.

We assume all tracks can contain the same amount of info.

• Even though we can clearly see the track in the center is way smaller than the one on the outside??? TODO

Disk Scheduling

Disk bandwidth: Totla number of bytes transferred, divided by total time between first request and completion of last transfer.

When a process need I/O, it issues a system call that specified:

• Whether the operation is input or output.
• Disk address for the transfer.
• Memory address for the transfer.
• Number of sectors to be transferred.

OS maintains a queue of request, per disk/device.

• Idle disks can immediately work, but busy disks need a queue.

Remember than optimization algorithms only make sense when a queue exists. TODO seems self evident, why the note

FCFS

Intrinsically fair, but generally not the fastest.

TODO We calculate absolute value of distance and add them all together to get total head movements (???)

TODO Copy diagram.

SSTF

Shortest Seek Time First selects request with minimum seek time from the current head position.

• Like SJF, can starve some requests.

TODO Copy diagram.

Most operating system use this.

SCAN / Elevator

Disk arm starts at one end of disk and moves to the other end, services requests as it reaches each cylinder, where the head movement is reverse and servicing continues.

• Can starve the requests at the other end of the disk.

TODO The above explanation sucks. It’s basically ping-ponging around.

TODO Copy diagram.

• Note that the direction is determined by the previous position. (TODO so when implementing this, do that to know the direction)