Mass-Storage

Question 1

What is the structure of a disk?

Answer 1

One-dimensional arrays of logical blocks (logic_blocks[])

Data addresses with logical block number (index)

Question 2

What makes a good disk?

Answer 2

• Speed (bandwidth)
• Speed (Access time, or latency)
• Reliability
• Power
• Cost
• Capacity

Question 3

What is disk scheduling?

Answer 3

Queue of pending requests created when multiple processes use the disk at once.

Question 4

What affects the performance of disks when it comes to disk scheduling?

Answer 4

Order in which requests are serviced.

Minimisation of head movement.

Fairness

Question 5

What are the advantages of FCFS Scheduling?

Answer 5

Simple

Fair - no chance of starvation

Question 6

What are the disadvantages of FCFS scheduling?

Answer 6

May involve excessive head movement as it doesn’t take into account other requests in the queue.

Question 7

What is SSTF Scheduling?

Answer 7

Shortest Seek-Time First Scheduling.

Next request processed is the one with the shortest seek time.

Try to service all requests in local area before moving, pick request with closest cylinder number.

Question 8

What are the disadvantages of SSTF Scheduling?

Answer 8

Starvation can occur. A request can be held up indefinitely if it far from local area being processed.

Question 9

What is SCAN scheduling?

Answer 9

Head continually scans disk from one end to the other and back (Elevator algorithm)

Requests are serviced as the head passes. Fair.

Non-uniform delays

Question 10

What is C-SCAN scheduling?

Answer 10

Variant on SCAN, returns to start immediately on reaching end (circular).

Area with most requests is start.

Question 11

What is LOOK / C-LOOK Scheduling?

Answer 11

Variants on SCAN / C-SCAN, only goes as far as final requests in each direction (IE Bounded)

Question 12

How do SSDs deal with requests?

Answer 12

FCFS policy, can’t use other scheduling policies as it doesn’t have a moving head.

Question 13

In what time are Reads and Writes on SSDs?

Answer 13

Read: Uniform
Write: Non-uniform
Some SSD schedulers merge only adjacent write requests.

Question 14

How do Block interfaces to SSD’s work?

Answer 14

Logical only.

SSDs run software that manage wear on components. Requesting a particular block doesn’t guarantee a hardware block address.

Question 15

What is deduplication?

Answer 15

Don’t store things twice on the drive.

Question 16

How do we check for duplication?

Answer 16

We use hashing, on both a per-file basis and block basis.

Question 17

What are the issues with per-file hashing?

Answer 17

Adding anything to a file breaks the hash similarity

Question 18

Is block hashing better than file hashing?

Answer 18

Yes but not in all cases.

Inserting any data not the size of a block breaks the similarity.

Requires more computational power.

Question 19

How does variable length hashing work?

Answer 19

Uses a rolling hash.

Resyncs the hash boundaries because the data is the same.

Shift resistant hashing.

Question 20

What issue exists with variables length hashing?

Answer 20

Hard to parallelise.

Question 21

What is RAID?

Answer 21

Many disks attached to a computer system.

Improves Read/Write performance and reliability.

Redundant Array of Inexpensive Disks

Question 22

What is the effect of Redundancy in raid?

Answer 22

Improves reliability as >1 copy of data.

However, this causes a storage overhead

Question 23

What is mirroring?

Answer 23

Duplicate each disk, single logical disk consists of 2 mirrored physical disks. Write to both, read from one.

Question 24

How does RAID affect parallelism?

Answer 24

Read request service rate doubled with mirroring, request can be serviced from either copy.

Transfer rate increased through striping.

Question 25

What is data striping

Answer 25

Data fragmented and each fragment written o a different disk.

Increases transfer rate, can read from both.

Question 26

What do each of the different RAID levels mean?

Answer 26

Different combinations of mirroring, striping and parity techniques

Cost/performance trade offs different for each

Question 27

What are the attributes of RAID 0?

Answer 27

Block-level striping, no redundancy but improved performance. Min 2 disks.

Question 28

What are the attributes of RAID 1?

Answer 28

Disk mirroring.
Min 2 disks.
- High storage overhead.

Question 29

What are the attributes of Raid 2?

Answer 29

Error Correcting Code organisation, correct single-bit errors and detect double-bit errors.

EEC stored on additional disks.

Question 30

What happens on read with a Raid 2 organisation?

Answer 30

Ever disk takes part in every I/O request. Data and ECC bits delivered to controller on read.

Question 31

What are the attributes of Raid 3?

Answer 31

Bit-interleaved parity.

Raid 2 but only single parity disk. Can detect read errors

Question 32

What are the attributes of Raid 4?

Answer 32

Block interleaved parity. Parity block stored on additional disk.

Block access involves a single disk, multiple I/O requests serviced in parallel.

Question 33

What are the attributes of Raid 5?

Answer 33

Block-interleaved distributed parity. Parity blocks distributed among disks.

Avoid bottleneck of single parity disk.

Question 34

What are the attributes of Raid 6?

Answer 34

Raid 5 but with two parity calculations, meaning it can handle two disk falilures.