1. Multiple Platters

2. Cylinders

3. Disks Disk Hardware (1)
Disk parameters for the original IBM PC floppy disk and a Western Digital WD hard disk

4. Disk Hardware (2)
Physical geometry of a disk with two zones
A possible virtual geometry for this disk

5. Disk Formatting (1)
A disk sector

6. An illustration of cylinder skew
Disk Formatting (2)
An illustration of cylinder skew

7. Improving Disk Speed
No interleaving
Single interleaving
Double interleaving

8. Disk Arm Scheduling Algorithms (1)
Time required to read or write a disk block determined by 3 factors
Seek time
Rotational delay
Actual transfer time
Seek time dominates
Error checking is done by controllers

9. Disk Arm Scheduling Algorithms (2)
Initial
position
Pending
requests
Shortest Seek First (SSF) disk scheduling algorithm

10. Disk Arm Scheduling Algorithms (3)
The elevator algorithm for scheduling disk requests

11. Timing of Disk I/O Transfer

12. RAID
Redundant Array of Independent Disks
Redundant Array of Inexpensive Disks
6 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

13. Data striped across all disks Round Robin striping Increase speed
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

14. Data is striped across disks 2 copies of each stripe on separate disks
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

15. Disks are synchronized Very small stripes
RAID 2
Disks are synchronized
Very small stripes
Often single byte/word
Error correction calculated across corresponding bits on disks
Multiple parity disks store Hamming code error correction in corresponding positions
Lots of redundancy
Expensive
Not used

16. RAID 3
Similar to RAID 2
Only one redundant disk, no matter how large the array
Simple parity bit for each set of corresponding bits
Data on failed drive can be reconstructed from surviving data and parity info
Very high transfer rates

17. 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

18. 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
N.B. DOES NOT MEAN 5 DISKS!!!!!

19. Two parity calculations Stored in separate blocks on different disks
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

20. RAID 0, 1, 2

21. RAID 3 & 4

22. RAID 5 & 6

23. Data Mapping For RAID 0

24. Computer Hardware Review
Structure of a large Pentium system

25. Two categories of I/O devices
Block devices
521bytes to 32,768 bytes
Character devices

26. Principles of I/O Hardware
Some typical device, network, and data base rates

27. Components of I/O devices
I/O devices have components:
mechanical component
electronic component
The electronic component is the device controller
may be able to handle multiple devices
Controller's tasks
convert serial bit stream to block of bytes
perform error correction as necessary
Copy data available to main memory

28. Three I/O methods revisited
Programmed I/O
Interrupt Driven I/O
DMA I/O

29. Steps in printing a string (Make the CPU do all the work)
Programmed I/O (1)
Steps in printing a string (Make the CPU do all the work)

30. Direct Memory Access (DMA)
Operation of a DMA transfer