What is the cache block size (in words)? How many entries does the cache have? What is the ratio between total bits required for such a cache implementation over the data storage bits?

INSTRUCTIONS TO CANDIDATES

ANSWER ALL QUESTIONS

• What is the cache block size (in words)?
• How many entries does the cache have?
• What is the ratio between total bits required for such a cache implementation over the data storage bits?
• The following table represents the byte-addressed cache references starting from power on. Time 0 is the first memory access, and time 15 is the most recent memory access.  Fill in the table by converting the address to binary, deriving the index number, and recording if the entry in the cache was replaced.

Only the bolded values (added by Sri) are worth marks. 

 

Time	Address	Address in Binary	Index	Cache Entry Was Replaced (Yes or No)
0	0
1	4
2	16
3	132
4	232
5	160
6	1024
7	30
8	140
9	3100
10	180
11	2180
12	3505	70
13	185
14	869
15	7041

 

• What is the hit ratio?
• List the final state of the cache, with each valid entry represented as a record of <index, tag, data>.  There may be extra rows, or blank rows.

b. Textbook question 5.8 with adjustment by Sri – 4 marks.
Based on section 5.5 in the textbook.

Mean Time Between Failures (MTBF), Mean Time To Replacement (MTTR), and Mean Time To Failure (MTTF) are useful metrics for evaluating the reliability and availability of a storage resource. Explore these concepts by answering the questions about devices with the following metrics.

Item	MTTF	MTTR	MTBF	Availability
Smart phone	3 years	1 day
Any desktop hard drive	6 years	2 days
Specific desktop hard drive (order through Amazon)	6 years	4 days
Enterprise SSD with onsite replacement warranty	167 years	4 hours

 

Calculate the MTBF for each of the devices in the table by filling in the MTBF column in the chart above.

Calculate the availability for each of the devices in the table by filling in the Availability column in the chart above.

3. a) What happens to availability as the MTTR approaches 0?

     b) Is this a realistic situation?

 4. a)   What happens to availability as the MTTR gets very high, i.e., a device is difficult to repair?

     b)    Does this imply the device has low availability? (Sri note: This is a real problem with NAS/SAN’s where a ‘drive’ is really a box with many drives in it, and fixing a SAN could require a specialist electronics technician to diagnose the board and solder replacement components on, this has happened at Trent before). 

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