LUN ID number. Is order important?

[vmproteau]

Virtual Center 2.5 Update 1; ESX 3.5 Update 1

Our virtual Infrastructure is growing and we started adding Hosts to existing ESX Clusters. The existing Hosts have ~10 LUNs presented and since they were built together, the LUN IDs match between them.

I just noticed that an engineer is getting ready to add another Host to this cluster and has presented the LUNs in a different order. The LUN IDs. on thi Host is different than the other 2.

Is this an issue?

[oreeh]

This is an issue!

LUNs should be zoned equally across all hosts in a cluster.

[beagle_jbl]

Without question the LUN ID's must match. IE: If LUN4 is the LUN with a 300GB VMFS partition for SQL servers, it had better be LUN 4 on every server in the cluster. If not, a virtual mushroom cloud will be produced when you try and vmotion VM's to that host.

[RParker]

I don't think that's the problem. I think the problem is this.

He has LUNS lets say the are in sequential order, LUN1 is ID 1, LUN2 is ID 2, etc..

Now some engineer is coming in and he is created 2 NEW LUNS LUN20 ID 4 and LUN21 ID 5..

He wants to know if order matters, and the answer is no. For easy of administration and a sane mind, yes. But ESX will assign LUN20 (it doesn't care what ID it has, so long as its not duplicated) from ID 4, and it will be presented as such. ALL the ESX hosts that can see ID 4 will see the matching LUN20 datastore.

That's not an issue, I believe that's what he wants to know.

[vmproteau]

Sorry if I wasn't clear. I've attached an image. I'm wanting to know if the Host I'm adding to the Cluster has to look exactly like this or if these can be rearranged?

For instance, Could the 5GB LUN be ID 6 on the new Host?

[RParker]

It can.. but then it becomes confusing. Why do you want to change the order?

It's just an ID, and if you tag a datastore with a naming convention, your order of datastores will be totally askew.. So are you sure you know what you want to accomplish by doing this?

Please explain.

[beagle_jbl]

Give me a moment to word this better.

[vmproteau]

RParker - I don't want to change the order, it's just that the engineer presented the LUNs to this new Host in a different order. When I ask him to redo it, I wanted to give him an definitive answer why- when he asks. Frankly, I wasn't sure if it mattered or not.

[beagle_jbl]

Sometimes one follows the best practices without fully understanding why. I am guilty. I knew the LUN ID's had to match but in truth I had no idea why.

Here's some documentation:

From SAN Config Guide pg 113.

Issues with Displaying the Same LUN IDs Across Hosts

Normally, a LUN appears with the same LUN ID to all hosts that access the LUN. On

some arrays, however, it may not be possible to display the LUN with the same LUN

ID across hosts. As a result, the ESX Server system incorrectly detects the LUN as a

snapshot and places it offline.

I think there are workarounds but.... if it's trivial for him to present you the same LUN IDs I would just do it.

[beagle_jbl · Accepted solution]

Page 60:

If you are using VMotion, DRS, or HA, make sure that both source and target hosts

for virtual machines can see the same LUNs with identical LUN IDs.

[vmproteau]

Perfect. I should have probably searched the SAN Configuration Guide but, the communities never let me down. I'm sure this thread will help someone.

Thanks for everyone's input.