Structure of a hard disk

Head = read head
Platter = magnetic recording media

Schematics of a hard disk

Structure of a platter

Sector = unit of data recording area on disk
Track = all sectors at same distance to center

Terminology: "cylinder"

Cylinder = all tracks at same distance to center

Disk block

Disk block = UNIT of read/write operation on disk

Data file

File = storage for user data
Directory entry is used to locate the data blocks of a file

File directory

File Directory is locate on (special) reserved tracks on the disk

How to access a data file (stored on disk)

Steps to access data on disk - step 1: move disk head to cylinder

Seek time = time needed for disk head to travel to cylinder

Steps to access data on disk - step 2: wait for sector

Rotational delay = waiting time for the desired sector to arrive

Steps to access data on disk - step 3: transfer data

Transfer time = time to data from disk platter to main memory

Disk sector access time (delay)

Disk access time (delay) = seek time + rotational delay + transfer time
(Delay is in the order of milli seconds)

Reducing data access time: storage strategy