# EMC Interview Questions

Posted by Lianamelissa on June 28th, 2018

Q. As a SAN administrator how will you tell your boss how many drives are required for a requirement?
I will use the formula:
Total Approximate Drives required = (RAID Group IOPS / (Hard Drive Type IOPS)) + Large Random I/O adjustment + Hot Spares + System Drives

Q. How to calculate HDD capacity?
Capacity = Heads X Cylinders X Sectors X Block Size

Q. You need to provision SAN storage with a certain IOPS. How will you find what kind of disks you need?
Input/output operations per second (IOPS) is the measure of how many input/output operations a storage device can complete within one second.
IOPS is important for transaction-based applications.
IOPS performance is heavily dependent on the number and type of disk drives.
To calculate IOPS of a Hard disk drive:
1
IOPS = —————————————
(Average Latency) + (Average Seek Time)
To calculate IOPS in a RAID:

Q. How will you calculate Max IOPS an HBA Port can generate to any LUN?
Max IOPS an HBA Port can generate to any LUN = (Device Queue Depth per LUN * (1 / (Storage Latency in ms/1000)))

Q. What is Q-Depth? How to calculate it?
The queue depth is the maximum number of commands that can be queued on the system at the same time.
Q is the Queue Depth =Execution Throttle= Maximum Number of simultaneous I/O for each LUN on a particular path to the Storage Port.
Calculation of the maximum queue depth: The queue depth is the number of I/O operations that can be run in parallel on a device.
Q = Storage Port Max Queue Depth / (I * L),
I is the number of initiators per storage port
L is the quantity of LUNs in the storage group.
T = P * q * L
T = Target Port Queue Depth
P = Paths connected to the target port
Q = Queue depth
L = number of LUN presented to the host through this port
Execution Throttle= (Maximum Storage Port Command Queue) / (Host Ports)

Q. How will you calculate number of drives required?
Total Approximate Drives required = (RAID Group IOPS / (Hard Drive Type IOPS)) + Large Random I/O adjustment + Hot Spares + System Drives

Q. If you know I/O load and IOPS, how will you calculate how many drives will be needed?
Total Approximate Drives = (RAID Group IOPS / (Hard Drive Type IOPS)) + Large Random I/O adjustment + Hot Spares + System Drives

Q. How will you calculate HDD Capacity?
Capacity = Heads X Cylinders X Sectors X Block Size

Q. What is relation between rotational speed and latency time?
The Rotational speed and latency time is related as follows:
Latency time = (1/((Rotational Speed in RPM)/60)) * 0.5 * 1000 milli seconds
Latency and RPM:
HDD
Spindle RPM         Average rotational latency [ms]
7,200                               4.17
10,000                             3.00
15,000                             2.00

Q. What SAN design you will choose? Why?
Core-Edge