Sistemas operacionais - execicios e respostas mass storage structure

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Mass
Storage
Structure

10

CHAPTER

Practice Exercises
10.1

Is disk scheduling, other than FCFS scheduling, useful in a single-user
environment? Explain your answer.
Answer:
In a single-user environment, the I/O queue usually is empty. Requests
generally arrive from a single process for one block or for a sequence of
consecutive blocks. In these cases, FCFS is an economicalmethod of disk
scheduling. But LOOK is nearly as easy to program and will give much
better performance when multiple processes are performing concurrent
I/O, such as when a Web browser retrieves data in the background while
the operating system is paging and another application is active in the
foreground.

10.2

Explain why SSTF scheduling tends to favor middle cylinders over the
innermostand outermost cylinders.
Answer:
The center of the disk is the location having the smallest average
distance to all other tracks. Thus the disk head tends to move away
from the edges of the disk. Here is another way to think of it. The
current location of the head divides the cylinders into two groups. If
the head is not in the center of the disk and a new request arrives, the
new requestis more likely to be in the group that includes the center
of the disk; thus, the head is more likely to move in that direction.

10.3

Why is rotational latency usually not considered in disk scheduling?
How would you modify SSTF, SCAN, and C-SCAN to include latency
optimization?
Answer:
Most disks do not export their rotational position information to the
host. Even if they did, thetime for this information to reach the
scheduler would be subject to imprecision and the time consumed by
the scheduler is variable, so the rotational position information would
become incorrect. Further, the disk requests are usually given in terms
35

36

Chapter 10 Mass-Storage Structure

of logical block numbers, and the mapping between logical blocks and
physical locations is verycomplex.
10.4

Why is it important to balance file system I/O among the disks and
controllers on a system in a multitasking environment?
Answer:
A system can perform only at the speed of its slowest bottleneck. Disks
or disk controllers are frequently the bottleneck in modern systems as
their individual performance cannot keep up with that of the CPU and
system bus. By balancing I/O amongdisks and controllers, neither an
individual disk nor a controller is overwhelmed, so that bottleneck is
avoided.

10.5

What are the tradeoffs involved in rereading code pages from the file
system versus using swap space to store them?
Answer:
If code pages are stored in swap space, they can be transferred more
quickly to main memory (because swap space allocation is tuned for
fasterperformance than general file system allocation). Using swap
space can require startup time if the pages are copied there at process
invocation rather than just being paged out to swap space on demand.
Also, more swap space must be allocated if it is used for both code and
data pages.

10.6

Is there any way to implement truly stable storage? Explain your
answer.
Answer:
Truly stable storagewould never lose data. The fundamental technique
for stable storage is to maintain multiple copies of the data, so that if
one copy is destroyed, some other copy is still available for use. But for
any scheme, we can imagine a large enough disaster that all copies are
destroyed.

10.7

It is sometimes said that tape is a sequential-access medium, whereas
a magnetic disk is arandom-access medium. In fact, the suitability
of a storage device for random access depends on the transfer size.
The term streaming transfer rate denotes the rate for a data transfer
that is underway, excluding the effect of access latency. By contrast, the
effective transfer rate is the ratio of total bytes per total seconds, including
overhead time such as access latency.
Suppose that, in a...
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