Guest iSCSI mapping vs Raw Device Mapping

Question

Dear All,

I have some LUNs which I map to a Windows server, is there an advantage to mapping the LUN via ESXi versus mapping with the iSCSI initiator on the guest?

Which method is Read-write more capable and Multipath?

Answer 1

Hi,

Thanks for posting on our forum!

Firstly, I need to determine your production environment, VMware or Hyper-V (RDM seems to be a VMware product)? From my perspective, the choice then, is whether you wish control and management of iSCSI LUNs via the ESXi host or via the guest OS. If the latter, ESXi is only aware of your storage traffic as network traffic - it treats it and understands it as just another IP packet on the VM network setup. If an RDM through the ESXi host, you get the advantage of managing the LUNs, multipathing, and connectivity via the hosts and/or vCenter, easing many aspects of LUN management. And I see you prefer to have multipath and more capable read-write function, so I may suggest RDM. But if you are in a VMware environment, I strongly suggest you to contact their engineers directly to gain more professional information on RDM.

Secondly, for our Microsoft, we have SMB multichannel to support clients who use our product (like Hyper-V) while have the same demand as yours. If you also use Hyper-V, you can take a look at this product:
https://learn.microsoft.com/en-us/previous-versions/windows/it-pro/windows-server-2012-r2-and-2012/dn610980(v=ws.11)

Finally, I found some articles comparing the pros and cons of Guest iSCSI mapping and Raw Device Mapping respectively:
https://www.teimouri.net/guest-connected-vs-raw-device-mapping-rdm/

https://support.liveoptics.com/hc/en-us/articles/229590427-RDM-vs-Guest-iSCSI-connections

https://www.parallels.com/blogs/ras/what-is-rdm/

Please note: Information posted in the given link is hosted by a third party. Microsoft does not guarantee the accuracy and effectiveness of information.

Thanks for your support! Besides, would you please help me Accept Answer. An accepted blog can be put on top of our forum, so that people who have a similar issue can get access to their solution more quickly.

BR,
Joan

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Answer 2

Hey,

As mentioned, both options will work and you will be able to configure multipathing either on ESXi level or on Windows VM. As for performance, you shouldn't see any difference, but I would still run tests before using the configuration. StarWind VSAN for vSphere can be used as an iSCSI storage for vSphere deployments. The following guide covers the configuration process:
https://www.starwindsoftware.com/resource-library/starwind-virtual-san-for-vsphere/

Cheers,

Alex Bykovskyi

StarWind Software

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