topic Why RDM? in VMware Solutions Discussions

Why RDM?

healthyfrog — Thu, 05 Jun 2025 04:52:47 GMT

1 Can anyone let me know in which circumstances RDM is required in Netapp environment ? Given RDM and VMDK is comparable from performance perspective, I am asking any particurlar netapp feature or best practise requires RDM.

2 I found the following from SnapDrive 6.5 guide http://www.netapp.com/us/media/tr-4000.pdf

Supports iSCSI initiators based on ESX, LUN migration with VMware® vMotion® , and RDM LUNs on virtual machines (VMs) hosted by ESX servers

Are both physical & Virtual RDM supported by SnapDrive 6.5

3 Anyone has experience to migrate RDM to VMDK without breaking existing NetApp features?

Thank you very much

Re: Why RDM?

SomBane — Fri, 13 Mar 2015 18:39:10 GMT

There are folks more qualified than me to answer this, but since I'm here and you have no replies... 🙂

First thing that I found helpful when I got started with NetApp storage was reviewing TR-3749 which focuses on storage best practices between NetApp and vSphere. There's a comparison table that shows the features of each type of storage protocol.

As far as question 1, I'm not aware of too many scenarios where you are required to use RDMs although 1 example is Microsoft clustering. Even though VMware seems to recommend VMDK as the default option, NetApp best practice guides for things like MSSQL and Exchange use RDMs, which I believe stems from integrating with SnapManager. As far as your query about SnapDrive, my understanding is that it uses physical mode only, but I haven't checked this issue out specifically in the last couple years.

I haven't attempted to migrate RDM to VMDK, but I will say that VMDK formatted disks won't have the SnapDrive management and operation features if you're reliant on them. But then I'm not sure what services you are running, so my comments are unfortunately a bit generic...

Re: Why RDM?

healthyfrog — Mon, 16 Mar 2015 04:12:18 GMT

Many thanks for your responding. It is very helpful.