How many paths for multipath in this situation ?

[TonyJK]

Currently, we have 4 paths for multipathing. We are using 2 HBAs / 2 FC Switchs with a EMC FC Storage Array.

One of my colleague suggests adding a ISL link between the switch to increase redundancy.

However, from my understanding, the number of path will become 8 (instead of 4). Is it correct ?

Is it a common practise to add a ISL link between 2 FC switches for redundancy ?

Thanks

[alex0 · Accepted solution]

Personally I don't see why it makes sense.

Why would you want to access Switch B via (Switch A+ISL link) when you ALREADY have a direct path to Switch B?

Also how does adding an ISL link between the switches increase redundancy? If a switch goes down, the ISL link is useless. If an HBA goes down, you already have paths to both switches from the remaining HBA (assuming you have 2 dual-port HBAs). Even with two single port HBAs, if you lose an HBA, so you only have one remaining HBA port on say switch A, why would you want to access Switch B via the ISL link... wouldn't it make more sense to just put data straight through Switch A?

Also, on ESX 3.5U2 the total number of paths on a server is 1024. With 8 paths per LUN, you will only be able to allocate 128 LUNs, even though ESX 3.5U2 supports up to 256 LUNs, you will be bottlenecked by the paths.

Perhaps a storage expert could wade in, because I'm not seeing ISL's making much sense from a VMware perspective.

[csoto]

I'm going to concur with Alex here. You've already narrowed the SPOFs, so ISL isn't worth the complication.

[depping]

I'm no storage expert either but same here... I don't see the point.

Duncan

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