Chapter 7, Problem 39E

Explanation of Solution

Given Data:

Capacity of HDD = 800GB

Cost of HDD = $300

Electricity cost = $0.10/kilowatt hour

Facilities cost = $0.01/GB per month

Active time = 25% = 0.25%

To Find:

Cost to store 8TB of data for 5 years.

Steps necessary to reduce the cost.

From Figure “7.15”:

Power requirement for read/write operation = 14.4 Watts

Idle power requirement = 9.77 Watts

Solution:

Cost to store 8TB data for 5 years:

Specifications table:

Active watts = 14.4 Watts (Given from figure “7.15”)

Idle watts = 9.77 Watts (Given from figure “7.15”)

Calculating idle percentage: Idle percentage is calculated by subtracting the active percentage from 1.

$\begin{array}{l}\text{Idle percentage = 1 - active percentage}\\ \text{Idle percentage = 1 - 0}\text{.25}\\ \text{Idle percentage = 0}\text{.75}\end{array}$

Filling specifications table:

Specifications
Hours/year	8,760
Cost per kWh	0.1
Active percentage	0.25
Active watts	14.4 Watts
Idle percentage	0.75
Idle watts	9.77 Watts

Facilities table:

Given that: Fixed cost/GB/month = 0.01

Calculating total cost per month: Total cost/month is calculated by multiplying fixed cost per GB per month and number of GB.

$\begin{array}{l}\text{Total cost/month = cost/GB }\times \text{ Number of GB}\\ \text{Total cost/month = \$0}\text{.01 }\times \text{ 8000}\\ \text{Total cost/month = \$}80\end{array}$

Calculating total number of months: Number of months is calculated by multiplying total number of years and months/ year.

$\begin{array}{l}\text{Number of months = Total years × months/year}\\ \text{Number of months = 5 years × 12 months}\\ \text{Number of months = 60 months}\end{array}$

Calculating total facilities cost: Total facilities cost is calculated by multiplying total cost per month and total number of months.

$\begin{array}{l}\text{Facilities cost = Cost/month }\times \text{ number of months}\\ \text{Facilities cost = \$80 }\times \text{ 60}\\ \text{Facilities cost = \$}4800\end{array}$

Filling facilities table:

Facilities
Fixed cost per GB per month	0.01
Number of GB	× 8,000
Total cost per month	$80
Number of months	60 months
Total facilities cost	$4800

Final table:

Given that: Hours active/year = 0.25 × 8760 = 2190hr

Calculating kWatts consumed active: It is calculated by multiplying number of hours active/year and power requirement for read/write operation:

$\begin{array}{l}\text{kWatts consumed active = hours active/yr × power for read/write}\\ \text{kWatts consumed active = 2190 hr × 14}\text{.4 W}\\ \text{kWatts consumed active = 31536 Wh}\\ \text{kWatts consumed active = 31}\text{.5 kWh}\end{array}$

Calculating idle/year: It is calculated by multiplying Hours/year and Idle percentage:

$\begin{array}{l}\text{Idle/year = hours/year × Idle percentage}\\ \text{Idle/year = 8760 × 0}\text{.75}\\ \text{Idle/year = 6570 hours}\end{array}$

Calculating kWatts consumed idle: It is calculated by multiplying idle/year and idle watts:

$\begin{array}{l}\text{kWatts consumed idle = idle/yr × idle watts}\\ \text{kWatts consumed idle = 6570 hr × 9}\text{.77 W}\\ \text{kWatts consumed idle = 64188 Wh}\\ \text{kWatts consumed idle = 64}\text{.2 kWh}\end{array}$

Calculating total kWatts: It is calculated by adding kWatts consumed active and kWatts consumed idle:

$\begin{array}{l}\text{Total kWatts = kW consumed active + kW consumed idle }\\ \text{Total kWatts = 31}\text{.5 kWh + 64}\text{.2 kWh}\\ \text{Total kWatts = 95}\text{.7 kWh}\end{array}$

Calculating Energy cost/year: It is calculated by multiplying Cost per kWh and total kWatts:

$\begin{array}{l}\text{Energy cost/year = cost/kWh }\times \text{ total kWh}\\ \text{Energy cost/year = \$0}\text{.1 }\times \text{ 95}\text{.7 kWh}\\ \text{Energy cost/year = }\$9.75\end{array}$

Calculating for 5 disks: It is calculated by multiplying Energy cost/year and 5:

$\begin{array}{l}\text{Cost for 5 disks = Energy cost/year × 5}\\ \text{Cost for 5 disks = \$9}\text{.75 × 5}\\ \text{Cost for 5 disks = \$47}\text{.85}\end{array}$

Calculating for 5 years: It is calculated by multiplying Energy cost/year for 5 disks and 5:

$\begin{array}{l}\text{Cost for 5 years = Energy cost/year for 5 disks }\times \text{ 5}\\ \text{Cost for 5 years = \$47}\text{.85 × 5}\\ \text{Cost for 5 years = \$239}\text{.25}\end{array}$

Calculating cost for 10 disks: It is calculated by multiplying $300 and 10:

$\begin{array}{l}\text{Cost for 10 disks = \$300 × 10}\\ \text{Cost for 10 disks = \$3000}\end{array}$

Calculating grand total: It is calculated by adding total facilities cost, energy cost for 5 years and disk cost for 10 disks:

$\begin{array}{l}\text{Grand total = Total facilities + Energy cost for 5 years + Disk cost for 10 years}\\ \text{Grand total = \$4800 + \$239}\text{.25 + \$3000}\\ \text{Grand total = \$8039}\text{.25}\end{array}$

Filling final table:

Hours active/yr	2190
kWatts consumed active	31.5 kWh
Hours idle/yr	6570 hours
kWatts consumed idle	64.2 kWh
Total kWatts	95.7 kWh
Energy cost/yr	$9.75
× 5 disks	$47.85
× 5 years	$239.25
+ disk cost $300 × 10	$3000
Grand total	$8039.25

Therefore, total cost to store 8TB data for 5 years is “$8039.25”.

Steps necessary to reduce the cost:

The important step to be followed in order to reduce the total cost is to ensure that the disk go in standby or sleep mode when it is not active. This might reduce the energy cost around 45% and total cost around 2%.