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:
(Total Workload IOPS * Percentage of workload that is read operations) + (Total Workload IOPS * Percentage of workload that is read operations * RAID IO Penalty)

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
This design includes redundant paths between switches,
As the fabric has two or more “core” switches in the center of the fabric,
This interconnects a number of “edge” switches. Hosts,
Storage and other devices connect to the free ports on the edge switches, in some cases also connect directly to the core switch.
It makes the fabrics highly scalable, without disruption to service.
I consider Core-Edge the most versatile forms of SAN design.

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