1. Copyright © 1998-2012 Curt Hill Disk Basics Structure and Characteristics of Secondary Storage

2. Copyright © 1998-2012 Curt Hill 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

3. 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

4. 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

5. Copyright © 1998-2012 Curt Hill Hard Disk Organization Track Sector Spindle Platter Access Arm Head

6. Copyright © 1998-2012 Curt Hill 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

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

8. 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

9. 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

10. 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

11. 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

12. 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

13. 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