CMSC 421 Operating Systems Lecture Notes
(c) 1995 Howard E. Motteler


DISKS
======


What's in a disk?
------------------

platter  - the metal (or plastic) disks

surface  - oxide on one or both sides of the disk

head     - magnetic coil that moves over the surface 

track    - path on surface under an (unmoving) head

sectors  - sections into which a track is divided

cylinder - a stack of tracks


[ Figures:

  -  generic hard disk

  -  floppy disk

  -  fixed head disk & drum ]



Disk hardware organization
---------------------------

Physical drive; base, platter, motor, heads

Motor control and analog electronics

Disk controller:

 - translates bit stream to bytes; in most cases does
   error checking and correction

 - translates surface, track, and sector values into
   head and motor commands

 - may define track 0 and sector 0 with low-level format

 - some disks can be formatted with varying sector sizes
   and/or track width

 - some newer disks have more sectors on outer tracks

Over time, the tendency is for controllers to become more
"intelligent", e.g., to include bad block management



Disk Addressing
---------------

A _disk block_ is usually just a sector.

(Sometimes disk blocks are groups of sectors;
we will use the terms synonymously, here)

A sector (or block) is the smallest addressable unit on
a disk

A disk block address has three components

   a = sector number
   b = surface
   c = cylinder (or track, since heads move together)

Suppose there are  

   s  sectors per track
   t  tracks per cylinder

Then the sector (or block) address is

   c * s * t  +  b * s  +  a


[ picture of numbered sectors ]


For example, suppose we have a disk with 

   s = 17 sectors per track and 
   t = 10 tracks 
   
The block with disk address 542 can be found at 

   c = cylinder =   542 / s * t  =  3

   b = surface  =  (542 mod s * t) / s  =  32 / 17  =  1

   a = sector   =  (542 mod s) =  15

Checking, we have

   3 * 17 * 10  +  1 * 17  +  15  =  542



Access Time
------------   

Three components:

  seek time - time to move heads to desired cylinder

  latency   - time for desired sector to come under head

  transfer time - time to read the desired sector(s)



Interleaving
-------------

In some cases (with slow processors and fast disks),
sectors can come off the disk faster than the processor
can read them.

If we just miss reading a sector, we must wait an entire
rotation to read it

In this case, it may be desirable to space out or
_interleave_ the sectors

Successively numbered sectors skip k sector positions in a
a k:1 interleave (or just a k-interleave)

Disks that may be interleaved often have a prime number of
sectors around a track


[ interleave examples ]