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Larry Schroeder
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SAS or SATA? Trying to create a RDM disk in VMware and it is failing with error message "Failed to create virtual disk: Function not implemented (2490377)"

Does it make a difference if you are using SATA or SAS disks when trying to create a RDM disk in VMware 6.7 with controller or other hardware.

We have similar deployments with Cisco UCS C240 M4 servers that have Cisco12G Modular Raid Controllers with a mix of spinning and SSD disks and have had no problem with creating RDM disks using vmkfstools. 

We have new servers and it is failing on the spinning disks.  I have cases open with both Cisco and VMware and they are blaming each other and offering no help.

The only differences that I can find on the servers are:

1. Servers that boot using UEFI fail to create RDM disks. Servers that boot using Legacy boot are ok. We need to use UEFI going forward.

2. Servers that have spinning SAS drives are failing to create RDM disks. SATA drives seem OK.

I think I would like to try SATA drives but I wanted to see what the Experts say before making any investment in different drives.




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Larry Schroeder

8/22/2022 - Mon

I found something related to the title of your question and it shows a solution.


As far as your main question about difference in disks someone may be along to help you with that.
Andrew Hancock (VMware vExpert PRO / EE Fellow/British Beekeeper)

Why RDM?

Is this to use dedicated hardware in a VM?
Larry Schroeder

We use software called StorMagic that acts similar to SVSAN. It uses an RDM as a shared Datastore that mirrors between hosts. I could use a VMDK but I am told there is a slight performance increase using RDM.
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Andrew Hancock (VMware vExpert PRO / EE Fellow/British Beekeeper)

We have seen this method, hit and miss over the years.

Just so I'm clear are you trying to do this, I wrote an article about it , 10 years ago, the purpose, VMware Admins, often use this method to present RAW disk to ESXi VMs. e.g. to build NAS which have direct access to the underlying storage rather than using a VMDK. (unlike VMware vSAN which requires the controller to support JBOD mode)

HOW TO: Add Local Storage (e.g. a SATA disk) as a Raw Disk Mapping (RDM) or Mapped RAW LUN to a virtual machine hosted on ESXi

I always thought his method was a "hack job" and not supported, and just a weird workaround.

Umm, RDM versus VMDK performance was a MYTH!

debunked 11 years ago!

RDM versus VMDK performance

Conclusion: VMFS and RDM have similar performance. Don’t choose RDM for performance.
Larry Schroeder

Yes, your article is what I am trying to do.

I am just curious as to why it doesn't work. I'll most likely end up using VMDK but then I will have a site that is different than others and I really like having everything the same. I imagine using RDM is more of a holdover back when there was a 2 TB limit with VMDK's. What was that version 5.0?? I am presenting a 6 TB drive. 
Andrew Hancock (VMware vExpert PRO / EE Fellow/British Beekeeper)

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Larry Schroeder

I guess I will go with VMDK going forward and not put any effort into finding out why RDM is no longer working for me.
Using VMDK is support in the software.

Thanks for all the insight and helping me to make a decision.

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