Chapter 7, Problem 40E

Explanation of Solution

Given Data:

Capacity of the disk = 800GB

Cost of SSD (800 GB) = $900

Cost of HDD (800 GB) = $300

Electricity cost = $0.10/kilowatt hour

Active time (both SSD and HDD) = 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 (HDD) = 14.4 Watts (Given from figure “7.15”)

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

Active watts (SSD) = 5 Watts (Given from figure “7.17”)

Idle watts (SSD) = 0.35 Watts (Given from figure “7.17”)

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	HDD	SSD
Hours/year	8,760	8,760
Cost per kWh	0.1	0.1
Active percentage	0.25	0.25
Active watts	14.4 Watts	5 Watts
Idle percentage	0.75	0.75
Idle watts	9.77 Watts	0.35 Watts

Final table:

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

Calculation for HDD:

Calculating kWatts consumed active (HDD): It is calculated by multiplying number of hour’s 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 (HDD): 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 (HDD): 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 (HDD): 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 (HDD): 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 (HDD): 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 cost for 10 disks (HDD): It is calculated by multiplying $300 and 10:

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

Calculation for SSD:

Calculating kWatts consumed active (SSD): It is calculated by multiplying number of hour’s active/year and power requirement for read/write operation:

$\begin{array}{l}\text{kWatts consumed active = hours active/yr × power for read/write}\\ \text{kWatts consumed active = 2190 hr × 5 W}\\ \text{kWatts consumed active = 10950 Wh}\\ \text{kWatts consumed active = 10}\text{.95 kWh}\end{array}$

Calculating idle/year (SSD): 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 (SSD): It is calculated by multiplying idle/year and idle watts:

$\begin{array}{l}\text{kWatts consumed idle = idle/yr × idle watts}\\ \text{kWatts consumed idle = 6570 hr × 0}\text{.35 W}\\ \text{kWatts consumed idle = 2299}\text{.5 Wh}\\ \text{kWatts consumed idle = 2}\text{.3 kWh}\end{array}$

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

$\begin{array}{l}\text{Total kWatts = kW consumed active + kW consumed idle }\\ \text{Total kWatts = 10}\text{.95 kWh + 2}\text{.3 kWh}\\ \text{Total kWatts = 13}\text{.25 kWh}\end{array}$

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

$\begin{array}{l}\text{Energy cost/year = cost/kWh }\times \text{ total kWh}\\ \text{Energy cost/year = \$0}\text{.1 }\times \text{ 13}\text{.25 kWh}\\ \text{Energy cost/year = }\$1.325\end{array}$

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

$\begin{array}{l}\text{Cost for 5 disks = Energy cost/year × 5}\\ \text{Cost for 5 disks = \$1}\text{.325 × 5}\\ \text{Cost for 5 disks = \$6}\text{.625}\end{array}$

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

$\begin{array}{l}\text{Cost for 10 disks = \$900 × 10}\\ \text{Cost for 10 disks = \$9000}\end{array}$

Filling final table:

Specifications	HDD	SSD
Hours active/yr	2190hr	2190hr
kWatts consumed active	31.5 kWh	10.95 kWh
Hours idle/yr	6570 hr	6570 hr
kWatts consumed idle	64.2 kWh	2.3 kWh
Total kWatts	95.7 kWh	13.25 kWh
Energy cost/yr	$9.75	$1.325
× 5 disks	$47.85	$6.625
× 10 disks	$3000	$9000

Cost comparison:

The difference between the energy cost for HDD and SSD is $41.23.

The difference between the energy cost for 10 disks for both HDD and SSD is $6000.

Therefore, it will take nearly 145 years for the difference in the cost between the SSD and traditional magnetic disks to offset by the cost of electricity saved by SSD.