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N-Step-SCAN

N-Step-SCAN is a computer science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand N-Step-SCAN rather than just read about it. In short: N-Step-SCAN (also referred to as N-Step LOOK) is a disk scheduling algorithm to determine the motion of the disk's arm and head in servicing read and write requests. It segments the request queue into subqueues of length N.

Key takeaways

  • N-Step-SCAN belongs to computer science; place it in that map before memorising details.
  • Learn the definition first, then one example that makes the definition concrete.
  • Connect N-Step-SCAN to a quantity you can measure, compute or draw — that is where exam questions come from.
  • Reproduce the core statement of N-Step-SCAN from memory before moving on to harder problems.

Reference excerpt

N-Step-SCAN (also referred to as N-Step LOOK) is a disk scheduling algorithm to determine the motion of the disk's arm and head in servicing read and write requests. It segments the request queue into subqueues of length N. Breaking the queue into segments of N requests makes service guarantees possible. Subsequent requests entering the request queue will not get pushed into N sized subqueues which are already full by the elevator algorithm. As such, starvation is eliminated and guarantees of service within N requests is possible. Another way to look at N-step SCAN is this: A buffer for N requests is kept. All the requests in this buffer are serviced in any particular sweep. All the incoming requests in this period are not added to this buffer but are kept up in a separate buffer. When these top N requests are serviced, the IO scheduler chooses the next N requests and this process continues. This allows for better throughput and avoids starvation.

Analysis N-Step-SCAN along with FSCAN prevents "arm stickiness" unlike SSTF, SCAN, and C-SCAN.

See also Other variations include:

SCAN - Elevator Algorithm FSCAN LOOK (and C-LOOK)

References

Worked examples

Example 1 — a first encounter with N-Step-SCAN

Start with the simplest possible case. Write down what N-Step-SCAN claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In computer science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to N-Step-SCAN before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about N-Step-SCAN ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of N-Step-SCAN

In research
N-Step-SCAN appears in computer science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry
Engineering practice reuses N-Step-SCAN in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom
N-Step-SCAN is common in secondary-school and first-year university syllabi. It links to neighbouring topics Disk scheduling algorithms, so understanding it makes those chapters shorter.
In everyday life
Look for N-Step-SCAN outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.
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How to study N-Step-SCAN in 20 minutes

  1. Read the reference excerpt below once, without taking notes.
  2. Close the page and write down what N-Step-SCAN means in your own words.
  3. Compare your version with the excerpt and mark what you missed.
  4. Work through the three examples above with pen and paper.
  5. Explain N-Step-SCAN out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is N-Step-SCAN in simple terms?

N-Step-SCAN (also referred to as N-Step LOOK) is a disk scheduling algorithm to determine the motion of the disk's arm and head in servicing read and write requests. It segments the request queue into subqueues of length N.

Why does N-Step-SCAN matter?

Because it connects several computer science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study N-Step-SCAN?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on N-Step-SCAN.

Tags

  • Disk scheduling algorithms

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