Disk Basics Structure and Characteristics of Secondary Storage

Why disks? • Computer memory has several problems: – Expensive – Volatile – Limitations on address space • Persistant data must be stored on disk or tape • The Operating System controls disk access – All disk access is somewhat platform dependent Copyright © 1998 -2012 Curt Hill

Disks • Rotating magnetic media • Flat, round platter of metal or plastic and covered with magnetic coating • Information stored as magnetized spots on the magnetic coating called bits • Access arms hold the heads – Move to read different areas Copyright © 1998 -2012 Curt Hill

Disk units • Organization of disk – Cylinders • Any area that can be read without moving head – Tracks • Concentric circles on surface • Disk rotates under head – Sectors • Pie shaped divisions on surface Copyright © 1998 -2012 Curt Hill

Hard Disk Organization Head Sector Track Access Arm Platter Copyright © 1998 -2012 Curt

Disks are DASD • Direct Access Storage Device • Any sector may be read directly • Disk address contains – Cylinder address • Forces moving heads – Track address • Selects a head – Sector address • Rotation of sector under head Copyright © 1998 -2012 Curt Hill

Hard Disk Drives Platters Read/Write head Copyright © 1998 -2012 Curt Hill

Disk Access Time • Seek time – Movement of access arm to correct cylinder • Rotational delay – Movement of disk to position correct sector under read/write head • Activation of appropriate read/write head • Transfer rate of data from disk to main memory Copyright © 1998 -2012 Curt Hill

Errors • The main problem is a head crash – The head physically touches the platter – Usually ruins the head and that area of the platter • Often catastrophic – Frequently the disk is unusable – Recovery services do exist, but are expensive and cannot guarantee success • Backup is required for mission critical data Copyright © 1998 -2012 Curt Hill

Disk Addresses • There are two forms – Cylinder/head/sector – Block number • Hardware always uses C/H/S • However, an IDE controller translates a block number into C/H/S internally – This is currently the most common disk data address Copyright © 1998 -2012 Curt Hill

Flash • A form of Electronically Erasable Programmable Read Only Memory (EEPROM) • Non-volatile • Capacities are now in the gigabyte range • Used as disk for One Laptop Per Child project Copyright © 1998 -2012 Curt Hill

RAID • Redundant Array of Independent Disks • Increase transfer rates by reading and writing data to several devices in parallel • Usually contains redundancy so that lost data from a disk crash can be reconstructed Copyright © 1998 -2012 Curt Hill

Storage Area Network (SAN) • A specialized, high-speed sub-network • Contains substantial storage capability in disk and tape • Connects to the CPUs (usually servers) in such a way as to appear local Copyright © 1998 -2012 Curt Hill