Fibre Channel (SAN)

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Occasional Contributor
Posts: 9
Registered: ‎11-17-2009

Oversubscription Calculation

Hi,

I was wondering if could ask what people think is the best way of working out oversubscription and how many ports to use in a trunk? I have read 2 methods about working it out based on physical number of ports divided by number of ports in the trunk or doing a similar exercise but based upon the actual port speeds.

I have to work something out for a client so we can trunk the new  switches to the current ones and so need to try and establish how many  ports to reserve for the trunk ports.

No matter which method you use, i assume you do all calculations based on the actual ports on the switches used by hosts/storage that will use the trunk and not just based on the whole switch. For example if only 20 hosts attached to my 5300 switch are going to be zoned in a way to use this trunk i would do all my oversubscription rate calculations on these 20 hosts.? Is that right.

Super Contributor
Posts: 260
Registered: ‎04-09-2008

Re: Oversubscription Calculation

Hi andrew.pilgrim

ISL oversubscription is calculated using

# servers / # ISL's

assuming connected devices are all the same speed and all ISL's are also of the same speed.

Use commands

switchshow -> To identify the speed of the connected devices

islshow -> To calculate the speed of the ISL between 2 switches

Then use the above formula to calc oversubscription.

What are the ideal oversubscription ratios??

Most successful SAN designs start with oversubscription ratio of 7:1 and reserve extra ports to support a future 3:1 ratio. High performance SAN's or conservative designs may start at 3:1

Use trunking (licensed feature) for better performance.

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