3. Magnetic Disk
Magnetic disks are the foundation of external memory on virtually all computer systems.
A disk is a circular platter constructed of nonmagnetic material, called the substrate, coated with a magnetizable material.
The substrate has been an aluminum material.
Glass substrates have been introduced.

4. Read and Write Mechanisms
Recording & 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

5. Read and Write Mechanisms
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

7. Data Organization and Formatting
Concentric set of rings, called tracks each track has the same width as the head. There are thousands of tracks per surface.
- 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 (track)

9. 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

11. Finding Sectors Must be able to identify start of track and sector
Format disk
- Additional information not available to user
- Marks tracks and sectors
An example of disk formatting is shown in Figure 6.4. In this case, each track contains 30 fixed-length sectors of 600 bytes each.
Each sector holds 512 bytes of data plus control information useful to the disk controller.

12. The ID field is a unique identifier or address used to locate a particular sector. The SYNCH byte is a special bit pattern that delimits the beginning of the field. The track number identifies a track on a surface.
The head number identifies a head, because this disk has multiple surfaces.
The ID and data fields each contain an error detecting
code.

15. Physical Characteristics
Head Motion
- Fixed head (one read write head per track and Heads mounted on fixed ridged arm)
- Movable head(one per surface and mounted on a movable arm).
Disk Portability
- Nonremovable disk (fixed )
- Removable disk (Can be removed from drive and replaced with
another disk, provides unlimited storage capacity, and easy data transfer between systems)

16. Physical Characteristics
Sides
- Single sided
- double (usually) sided
Platters
- Single platters
- multiple platter (One head per side, heads are joined and aligned, aligned tracks on each platter form cylinders and data is striped by cylinder:
1. Reduces head movement
2. Increases speed (transfer rate)

19. Physical Characteristics
Head mechanism
- Contact (Floppy disk) 8”, 5.25”, 3.5”
Small capacity up to 1.44Mbyte (2.88M never popular)
Slow, universal, cheap
- Fixed gap
- Flying (Winchester)
Developed by IBM in Winchester (USA), Sealed unit
One or more platters (disks), Very small head to disk gap universal, cheap, Fastest external storage
Getting larger all the time. 250 Gigabyte now easily available

20. RAID Redundant Array of Independent Disks (RAID
Redundant Array of Inexpensive Disks
7 levels in common use
Not a hierarchy
Set of physical disks viewed as single logical drive by O/S
Data distributed across physical drives
Can use redundant capacity to store
parity information

21. RAID These levels share three common characteristics:
1. RAID is a set of physical disk drives viewed by the operating system as a single logical drive.
2. Data are distributed across the physical drives of an array in a scheme known as striping, described subsequently.
3. Redundant disk capacity is used to store parity information, which guarantees data recoverability in case of a disk failure.
The details of the second and third characteristics differ for the different RAID levels. RAID 0 and RAID 1 do not support the third characteristic.

22. RAID 0 No redundancy Data striped across all disks
Round Robin striping
Increase speed
- Multiple data requests probably not on same disk
- Disks seek in parallel
- A set of data is likely to be striped across multiple disks

24. RAID 1 Mirrored Disks Data is striped across disks
2 copies of each stripe on separate disks
Read from either
Write to both
Recovery is simple
- Swap faulty disk & re-mirror
- No down time
Expensive

26. RAID 2 RAID levels 2 and 3 make use of a parallel access technique.
In a parallel access array, all member disks participate in the execution of every I/O request
the individual drives are synchronized so that each disk head is in the same position on each disk at any given time.
RAID 2 requires fewer disks than RAID 1

27. RAID 3 RAID 3 requires only a single redundant disk.
Employs parallel access, with data distributed in small strips
Can achieve very high data transfer rates.
Only one I/O request can be executed at a time

28. RAID 4 Each disk operates independently Good for high I/O request rate
Large stripes
Bit by bit parity calculated across stripes on each disk
Parity stored on parity disk

30. RAID 5 Like RAID 4 Parity striped across all disks
Round robin allocation for parity stripe
Avoids RAID 4 bottleneck at parity disk
Commonly used in network servers

31. RAID 6 Two parity calculations
Stored in separate blocks on different disks
User requirement of N disks needs N+2
High data availability
- Three disks need to fail for data loss
- Significant write penalty, because each write affects two parity blocks.