ESXi 5.1 Windows 2008 Failover Cluster

I have two Windows 2008 R2 VM's setup with RDM's in a Failover cluster. When I present the Lun to the VM I set the PSP to MRU. I noticed when I go to Storage -> Devices some Lun are showing RR while other are MRU.

I have also noticed when I click on some LUN in the Manage Paths dialog hags on "Loading" for several minutes.

This is a large cluster for our Core DB. I am not seeing any disk latency alerts yet I am still very nervous about this since VMware states only  MRU is supported with 5.1. I have already set the P.R reservations....

I know you can change the PSP on the fly but nervous..

Thoughts on this anyone or am I just being to gun shy? What would happen if I leave it as is? User are not complains of performace issues?
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Andrew Hancock (VMware vExpert / EE MVE^2)VMware and Virtualization ConsultantCommented:
Simple, if you leave it as is, VMware will not support it, if you ever need to escalate it to them for support.

But, have you ever tested, multipath, and path failure, this would be my concern.

If you had a path outage, it may take several minutes, and the hang looking at paths, does not seem correct.
compdigit44Author Commented:
I totally understand...

I am trying to understand WHY the Manage Paths dialog box takes minutes to open on some LUNs that are already set to MRU. Have you seen this before
compdigit44Author Commented:
I believe I figured out my own problem. All Lun on the active node VM had the PR reservation set but the passive node does not which I believe explains why the "Manage Paths" dialog box hangs.

Here is my question. I am using MRU and not seeing any performance issues what is the down side of not settings this?
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Andrew Hancock (VMware vExpert / EE MVE^2)VMware and Virtualization ConsultantCommented:
RR is what you should be normally using and is supported for Microsoft Failover Clusters with ESXi.

The downside of incorrect path policy, is loss of paths. Do you know the difference between MRU and RR ?

But it does depend on your SAN, and what your SAN vendor states is correct to use.
compdigit44Author Commented:
Hi Hancock... Thanks for the reply. Yes I do the the difference between RR and MRU and with 5.1 only MRU is supported which is what we are using. My questions is that I noticed some LUN have the PR reservation setting to false which causes the Managed Paths page to hang on Loading. My question is if we are not seeing any problems in regards to disk latency what are the problems of leaving this until our next maintenance window
Andrew Hancock (VMware vExpert / EE MVE^2)VMware and Virtualization ConsultantCommented:
compdigit44Author Commented:
Thank you for your patience with me Hancock.. So just as long as my PSP settings are correct for my version of ESXi and environment leave the PR reservation as false will pose no problem correct?  Then why does Vmware recommend setting it to true..
Andrew Hancock (VMware vExpert / EE MVE^2)VMware and Virtualization ConsultantCommented:
Because that is the supported configuration. I assume they must have had feedback or support issues, and thus change their KB to suit.

like RR is the only supported configuration for Microsoft Failover Cluster.

Yes it works with MRU, but it's supported only for RR, so if you had any issues, and escalated it to support, they would state, ah, well  you need to ensure you have a supported environment, and you must set this to true, and then if you have any issues, we will investigate the issue.

Same discussion and agrument, if you had unsupported firmware in HBA, and SAN controllers!

There is supported, and supported configurations....

PRs or Persistent Reservations feature heavily with LUNs, with clusters for locking, failover and takeover of LUNs, used in cluster storage configurations.

if you dig into your logs, vmkernel.log and look at failover of your cluster, there could be a danger of delayed, locking/unlocking of LUNs, and this delay could cause timeouts...

Unsupported Persistent Reservation Command - can be seen in the logs.

So test, failover, drop paths, but this may all be too late if in Production, but all these should have been done at implementation.

and look at the logs, in realtime, to see if any issues with storage failover.

e.g. we spend about 2 weeks, in test and implementation testing failovers before procduction.

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