->Magnetic Disk
+ RAID
+ Removable
->Optical
+ CD-ROM
+ CD-Recordable (CD-R)
+ CD-R/W
+ DVD
->Magnetic Tape
1. MAGNETIC DISK
->Disk substrate coated with magnetizable material (iron oxide…rust)
->Substrate used to be aluminium
->Now glass
. Improved surface uniformity
- Increases reliability
. Reduction in surface defects
- Reduced read/write errors
. Lower flight heights (See later)
. Better stiffness
. Better shock/damage resistance
READ AND WRITE MECHANISMS
. Recording and retrieval via conductive coil called a head
. May be single read/write head or separate ones
. During read/write, head is stationary, platter rotates
. Write
- Current through coil produces magnetic field
- Pulses sent to head
- Magnetic pattern recorded on surface below
. Read (traditional)
- Magnetic field moving relative to coil produces current
- Coil is the same for read and write
. Read (contemporary)
- Separate read head, close to write head
- Partially shielded magneto resistive (MR) sensor
- Electrical resistance depends on direction of magnetic field
- High frequency operation
+ Higher storage density and speed
Inductive Write MR Read
Data Organization and Formatting
-> Concentric rings or tracks
- Gaps between tracks
- Reduce gap to increase capacity
- Same number of bits per track (variable packing density)
- Constant angular velocity
-> Tracks divided into sectors
-> Minimum block size is one sector
-> May have more than one sector per block
DISK DATA LAYOUT
Disk Velocity
-> Bit near centre of rotating disk passes fixed point slower than bit on outside
of disk
-> Increase spacing between bits in different tracks
-> Rotate disk at constant angular velocity (CAV)
- Gives pie shaped sectors and concentric tracks
- Individual tracks and sectors addressable
- Move head to given track and wait for given sector
- Waste of space on outer tracks
. Lower data density
-> Can use zones to increase capacity
- Each zone has fixed bits per track
- More complex circuitry
Disk Layout Methods Diagram
Finding Sectors
-> Must be able to identify start of track and sector
-> Format disk
- Additional information not available to user
- Marks tracks and sectors
ST506 format (old!)
Characteristics
-> Fixed (rare) or movable head
-> Removable or fixed
-> Single or double (usually) sided
-> Single or multiple platter
-> Head mechanism
- Contact (Floppy)
- Fixed gap
- Flying (Winchester)
Fixed/Movable Head Disk
->Fixed head
- One read write head per track
- Heads mounted on fixed ridged arm
->Movable head
- One read write head per side
- Mounted on a movable arm
Removable or Not
->Removable disk
- Can be removed from drive and replaced with another disk
- Provides unlimited storage capacity
- Easy data transfer between systems
->Nonremovable disk
- Permanently mounted in the drive
Multiple Platter
->One head per side
->Heads are joined and aligned
->Aligned tracks on each platter form cylinders
->Data is striped by cylinder
- reduces head movement
- Increases speed (transfer rate)
Cylinders
