UnderStanding why MSCS in Vmware Requires RDM's

Posted on 2014-03-18
Last Modified: 2014-03-21
I understand you need to use a RDM in the following situations.

1) A Vm that requires an extremely large hard drive greater that 2TB
2) A VM that needs to use SAN tools
3) Running MSCS

But what technical reason does MSCS require an RDM.

I have search the internet and cannot find a answer so I am turning to toughs who are more knowledge than I.... :o)
Question by:compdigit44
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by:Andrew Hancock (VMware vExpert / EE MVE^2)
Andrew Hancock (VMware vExpert / EE MVE^2) earned 250 total points
ID: 39938507
It comes down to what Microsoft will Support if you should have issues when you contact the support department. They will only entertain you if using RDM or RAW.

Because its physical. Eg VM accesses a physical device as the LUN is passed through RAW.

Software iSCSI can also be used, it works...its up to organisations to access the Risk if you need to discuss with Msoft if you have issues.

SCSI reservations and locking have been issues in the past.
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ID: 39938508
Also VMDKs can now be up to 62TB so no RDM requirement...if you want a super size disk.
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Author Comment

ID: 39939344
Thanks for your response so there is not other technical reason why a RDM is required for MSCS except for certified support????

So when you map a LUN to a physical server it is a RAW mapping correct?

Has anyone tried to setup MSCS using VMDK?

There has to be a least one technical reason why MS still requires this besides from the support aspect..
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Accepted Solution

gheist earned 250 total points
ID: 39940662
Yes, you can share VMDK as mscs quorum
VAAI accelerated SAN/NAS recommended, but no problem without one.
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Author Comment

ID: 39941001
Interesting I wonder if times goes on if MS will change there stance on this.
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Expert Comment

ID: 39941085
Microsoft support is pay-per-event anyway...

See this:
VMWare KB1037959
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ID: 39941426
Correct when you map a LUN to VM it's RAW.

Virtual bus sharing works for VMDK.

Again access the risk we no very few clients that log support calls with Microsoft.
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Author Comment

ID: 39942213
I read the VMware KB you posted earlier. At the end of the document it states the following.

"VMware vSphere 5.5 provides complete support for 2012 failover clustering"

Does this mean VMware / MS support a Failover cluster running on a non-RDM?
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ID: 39944383
In 5.5 there are new enhancements for Failover Clustering in 2012 in which RDM do not need to be used eg iSCSI

See this kb
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Expert Comment

ID: 39945986
Microsoft supports hyper-v virtualisation, for other stuff you contact virtualisation vendor.

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