CS 4432 lecture #3 1

CS4432: Database Systems II

Lecture #3

Using the Disk, and Disk Optimizations

Professor Elke A. Rundensteiner

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Thus far :

• Hardware: Disks• Architecture: Layers of Access• Access Times and Abstractions

CS 4432 lecture #3 3CS 245 Notes 2 3

Optimizations (in controller or O.S.)

• Disk Scheduling Algorithms– e.g., elevator algorithm

• Larger Buffer• Pre-fetch• Disk Arrays• Disk Cache

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One Simple Idea : Prefetching

Problem: Have a File» Sequence of Blocks B1, B2

Have a Program» Process B1» Process B2» Process B3

...

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Single Buffer Solution

(1) Read B1 Buffer(2) Process Data in Buffer(3) Read B2 Buffer(4) Process Data in Buffer ...

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Say P = time to process/blockR = time to read in 1 blockn = # blocks

Single buffer time = n(P+R)

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Question: Could the DBMS know something

about behavior of such future block accesses ?

What if: If we knew more about the

sequence of future block accesses, how could we do better ?

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Idea : Double Buffering/Prefetching

Memory:

Disk: A B C D GE F

A B

done

process

AC

process

B

done

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Say P R

What is processing time now?

P = Processing time/blockR = IO time/blockn = # blocks

• Double buffering time = ?

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Say P R P = Processing time/blockR = IO time/blockn = # blocks

• Double buffering time = R + nP

• Single buffering time = n(R+P)

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Disk Arrays• RAIDs (various flavors)• Block Striping• Mirrored

logically one disk

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On Disk Cache

P

M C ......

cache

cache

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Block Size Selection?

• Question : Do we want Small or Big Block Sizes ?

• Pros ?• Cons ?

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Block Size Selection?• Big Block Amortize I/O Cost

– For seek and rotational delays are reduced …

• Big Block Read in more useless stuff!

and takes longer to read

Unfortunately...