Answer #1

1)

Disk scheduling is done by operating systems toschedule I/O requests arriving for the disk. Disk scheduling isalso known as I/O scheduling.

Disk scheduling is important because:

Multiple I/O requests may arrive by different processes andonly one I/O request can be served at a time by the diskcontroller. Thus other I/O requests need to wait in the waitingqueue and need to be scheduled.
Two or more request may be far from each other so can result ingreater disk arm movement.
Hard drives are one of the slowest parts of the computer systemand thus need to be accessed in an efficient manner.

There are many Disk Scheduling Algorithms but before discussingthem let’s have a quick look at some of the important terms:

Seek Time:Seek time is thetime taken to locate the disk arm to a specified track where thedata is to be read or write. So the disk scheduling algorithm thatgives minimum average seek time is better.
Rotational Latency: Rotational Latency is thetime taken by the desired sector of disk to rotate into a positionso that it can access the read/write heads. So the disk schedulingalgorithm that gives minimum rotational latency is better.
Transfer Time: Transfer time is the time totransfer the data. It depends on the rotating speed of the disk andnumber of bytes to be transferred.
Disk Response Time: Response Time is theaverage of time spent by a request waiting to perform its I/Ooperation. Average Response time is the response time ofthe all requests. Variance Response Time is measure of howindividual request are serviced with respect to average responsetime. So the disk scheduling algorithm that gives minimum varianceresponse time is better.
Disk Scheduling Algorithms
FCFS: FCFS is the simplest of all the DiskScheduling Algorithms. In FCFS, the requests are addressed in theorder they arrive in the disk queue.
Advantages:
Every request gets a fair chance
No indefinite postponement
Disadvantages:
Does not try to optimize seek time
May not provide the best possible service
SSTF: In SSTF (Shortest Seek Time First),requests having shortest seek time are executed first. So, the seektime of every request is calculated in advance in the queue andthen they are scheduled according to their calculated seek time. Asa result, the request near the disk arm will get executed first.SSTF is certainly an improvement over FCFS as it decreases theaverage response time and increases the throughput of system.

2)

FCFS Disk Head SchedulingAlgorithm

It works well under light load (few storagedisk accesses) and is the primaryalgorithm used withSolid-State Drives.

Linux provides several I/O schedulers that allow one to tuneperformance for your use patterns. Traditionally disk schedulingalgorithms have been designed for rotational drives whererotational latency and drive head movement latency need to be takeninto consideration, hence the need for complex I/O schedulers.However a SSD does not suffer from the HDD latency issues - doesthis mean the default I/O scheduler choice needs to bere-thought?

I decided to benchmark a SanDisk pSSD to see the behaviour ofthe different I/O schedulers using the following tests:

a) untar kernel source

b) sync

c) copy kernel source tree

d) sync

e) copy tar file

f) sync

g) rm -rf kernel source tree

h) rm -rf copy of source tree

i) sync

For each of the I/O schedulers I ran the above sequence of tests3 times and took an average of the total run time. My results areas follows:

cfq: 3m59s,

noop: 3m25s,

anticipatory: 3m51s,

deadline: 3m42s

So it appears the default cfq (Completely Fair Queuing)scheduler is least optimal and the noop scheduler behaves best.According to the noop wikipedia entry the noop scheduler is thebest choice for solid state drives, so maybe my test results can betrusted after all :-)

SSDs do not require multiple I/O requests to be re-ordered sincethey do not suffer from traditional random-access latencies, hencethe noop scheduler is optimal.

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<Operating System>1. Which scheduling objectives from module(CPU Scheduling) wouldalso apply to disk scheduling?2....

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