Question 1

Answer the following questions:
(a) What is the average time to read or write a 512-byte sector for a typical disk rotating at 7200 RPM? The advertised average seek time is 8ms, the transfer rate is 20MB/sec, and the controller overhead is 2ms. Assume that the disk is idle so that there is no waiting time.
(b) A program repeatedly performs a three-step process: It reads in a 4-KB block of data from disk, does some processing on that data, and then writes out the result as another 4-KB block elsewhere on the disk. Each block is contiguous and randomly located on a single track on the disk. The disk drive rotates at 7200RPM, has an average seek time of 8ms, and has a transfer rate of 20MB/sec. The controller overhead is 2ms. No other program is using the disk or processor, and there is no overlapping of disk operation with processing. The processing step takes 20 million clock cycles, and
the clock rate is 400MHz. What is the overall speed of the system in blocks processed per second assuming no other overhead?

Accepted Answer

a. Disk Access Time = seek time + rotational delay + transfer time + controller overhead
= 8 + (0.5*60*1000/7200) + (512/20*220)*1000 + 2
= 14.17 ms
b. Disk Read Time for a 4KB block
= seek time + rotational delay + transfer time + controller overhead
= 8 + (0.5*60*1000/7200) + (4*1024/20*220)*1000 + 2
= 14.17 ms
Processing Time = 20 * 106 * (1/(400*106))
= 1/20 = 0.05 s = 50 ms
Disk Write Time for 4 KB block = 14.17 ms
Total time to completely process a 4 KB block = 2*14.17 + 50 = 78.34 ms Number of blocks processed per second = 1000/78.34 = 12.76

Question 2

If a system contains 1,000 disk drives, and each of them has a 800,000 hour MTBF, how often a drive failure will occur in that disk system? Could you give some idea to improve that? And why will your idea work?

Accepted Answer

MTBF (array) = MTBF (one disk) / Number of disks in array So, answer is 800,000 / 1000 = 800 hrs
Set them up in a RAID 5 configuration which involves distributing the data and parity bits across the disk drives. This way even when one drive fails, the parity information enables the disk array to continue operation and rebuild the failed drive online once it has been replaced. Now, the likelihood that a second drive will fail before the failed drive is restored is pretty low. Hence the reliability can be shown to increase dramatically. To further improve the reliability, one can have a spare disk which is hot-swapped with the failed disk in order to reduce the restoration time.

Question 3

In Figure 6.9, PCI-E connections are available from both the north bridge and the south bridge. What are the advantages and disadvantages to attaching devices to the PCI-E connections on the north and south bridges?

Accepted Answer

The advantages of connecting PCI-E to the north bridge is that the interface is 'closer' to the processor and memory. That is to say, fewer hops from chip-to-chip must take place for data exchange, and as a result less competition for the data. This is illustrated in figure 6.9---PCI-E on north bridge has 4GB/sec bandwidth, though PCI-E on south bridge has 1GB/sec. This is also a disadvantage of the south bridge.
The advantage of connecting PCI-E to the south bridge is that, by lowering the priority of the PCI-E ports, we have raised the priority of the memory-to-processor connection. In many circumstances, memory throughput is of higher priority than I/O devices.

Question 4

A given computer system includes a hard disk with direct memory access (DMA).
(a) Suppose a user application needs to change a single byte within a disk block. Sketch, in order, all communications that must take place between the processor and the hard drive to complete this operation.

Accepted Answer

The answer of A given computer system includes a hard...

Question 5

Imagine that you are proposing a new disk IO benchmark for transaction processing, what sort of experiments would you perform to show that your benchmark's results are meaningful. Imagine that you are reviewing a paper introducing a new disk IO benchmark for transaction processing. What sort of subtle flaws would you search for?

Accepted Answer

The answer of Imagine that you are proposing a new...

Question 6

Which of the following would be an acceptable transport medium for real-time transmission of human voice data? Which would be "overkill"?
· 56.5Kbps modem
· 100 Base-T Ethernet connection
· 802.11b wireless connection.

Accepted Answer

The answer of Which of the following would be an...

Question 7

In Figure 6.8 which fields are correlated with each other? Why do these correlations exist?

Accepted Answer

The answer of In Figure 6.8 which fields are correlated...

Question 8

Discuss why RAID 3 is not suited for transaction processing applications. What kind of applications is it suitable for and why?

Accepted Answer

The answer of Discuss why RAID 3 is not suited...

Question 9

Section 6.7 focuses on transactional processing as an example of a disk IO intensive application. Give another example of a disk IO intensive application compare and contrast the performance requirements and consider how different disk implementations (magnetic media, flash memory, or MEMS device) can be more or less appropriate for different applications.

Accepted Answer

The answer of Section 6.7 focuses on transactional processing as...

Question 10

75 that hard disk block a+1 will be read soon afterwards. Given that an application has requested a read of a single disk block, the OS can expect the application to read subsequent blocks later. If the OS will pursue a strategy of reading n blocks at a time, analyze how the OS can choose this n in order to minimize the expected read time.

Accepted Answer

The answer of 75 that hard disk block a+1 will...

Question 11

What is the average time to read a 512 byte sector for Seagate ST31000340NS in Figure 6.5? What is the minimum time? Assume that the controller overhead is 0.2 ms, and the disk is idle so that there is no waiting time.

Accepted Answer

The answer of What is the average time to read...

Question 12

Mention two advantages and disadvantages for using a single bus as a shared communication link between memory, processor and I/O devices.

Accepted Answer

The answer of Mention two advantages and disadvantages for using...

Question 13

Give a concise answer to each of the following questions. Limit your answers to 20-30 words.
(a) What is memory mapped I/O?
(b) Why is DMA an improvement over CPU programmed I/O?
(c) When would DMA transfer be a poor choice?

Accepted Answer

The answer of Give a concise answer to each of...

Question 14

Suppose we have two different I/O system A and B. A has data transfer rate: 5KB/s and has access delay: 5 sec. While B has data transfer rate: 3 KB/s and has access delay: 4 sec. Now we have a 3M I/O request, taking performance into consideration, which I/O system will you use? What about for a 3KB request?

Accepted Answer

The answer of Suppose we have two different I/O system...

Question 15

What is the bottleneck in the following system setup, the CPU, memory bus, or the disk set?
$\bullet$The user program continuously performs reads of 64KB blocks, and requires 2 million cycles to process each block.
$\bullet$The operating system requires 1 million cycles of overhead for each I/O operation.
$\bullet$The clock rate is 3GHz.
$\bullet$The maximum sustained transfer rate of the memory bus is 640MB/sec
$\bullet$The read/write bandwidth of the disk controller and the disk drives is 64MB/sec, disk average seek plus rotational latency is 9ms.
$\bullet$There are 20 disks attached to the bus each with its own controller. (Assume that each disk can be controlled independently and ignore disk conflicts.)

Accepted Answer

The answer of What is the bottleneck in the following...

Question 16

7 before the first wear out if wear leveling working ideally?

Accepted Answer

The answer of 7 before the first wear out if...

Question 17

Disk Technology. Suppose we have a magnetic disk (resembling an IBM Microdrive) with the following parameters:
  Answer the following questions. (Note: you may leave any answer as a fraction.)
(a) What is the average time to read a single sector?
(b) What is the average time to read 8 KB in 16 consecutive sectors in the same cylinder?
(c) Now suppose we have an array of 4 of these disks. They are all synchronized such that the arms on all the disks are always on the same sector within the track. The data is striped across the 4 disks so that 4 logically consecutive sectors can be read in parallel. What is the average time to read 32 consecutive KB from the disk array?

Accepted Answer

The answer of Disk Technology. Suppose we have a magnetic...

Exam 6: Storage and Other Io Topics