Does VMware have the same IO limitations with virtual disks as Hyper-V for SQL Server?

With VMWare do the VMDK files (virtual disks) have the ability to create multiple paths the the SAN controller? Allow me to provide context and background as to why I am asking.

I was recently on a project with Hyper-V and we were migrating SQL Server to a Hyper-V cluster. The customer had purchased a more powerful set of hardware for the hyper-V environment and the thought was, we are giving 2x the amount of hardware to our new SQL Servers, so we will get 2x the performance.

We fought issues for weeks upon weeks, and we finally found out that when using Hyper-V virtual disks (VHDX) files, the VHDX files have a limitation as to the amount of SCSI channels they can establish to the SAN controller. (I am not a SAN guy, so I am paraphrasing) We determined this to be true by adding a second data file to the SQL Server database and moving that data file to a new VHDX drive and the performed the backup in half the time. After speaking with Nutanix, they stated that Windows Hyper-V should directly attach to the LUNs therefore bypassing the hypervisor for the storage layer. The OS and C drive remain on the hypervisor with a VHDX file.

The VM architect stated that this was a problem with only Hyper V and the VMware does not have this problem of creating multiple IO paths from the VMDK files to the SAN controller. I am no SAN admin, but this is hard for me to understand as I think if all my SQL data files (*.MDFs) are inside a virtual container, there must be limitations on this container limiting the amount of IO my SQL data files get.

I am now in another project where the VMware architect has presented virtual disk to the SQL Server, so before I go down this same road, I just want to know if VMware does this better than Hyper-V and will actually allow multiple IO paths from the VMDK files to the SAN controller allowing my SQL data files to leverage parallel IO.

Thanks in advance experts!
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Andrew Hancock (VMware vExpert / EE Fellow)VMware and Virtualization ConsultantCommented:
It seems the discussions is the difference between RAW (RDM) LUNs and Virtual Disk containers (e.g. VMDK or VHD)

does this sound like the discussions, e.g. presenting the SAN LUN direct to the VM?
sqlagent007Author Commented:
Correct. We found with Hyper-V that we gained acceptable performance by having the SAN / VM team present RAW LUNs to the Guests. When we used VHD files in Hyper-V the performance was horrible for SQL Server. Once we started creating multiple VHDs and striping the SQL Server data files across multiple VHDs the performance improved. However we still ran into issues where the Windows OS was seeing 20K ms disk latency and the Nutanix show less than 3ms latency.

Now I am on a project and the VMware guy is telling me that this will NOT be an issues because VMware does this so much better than Hyper-v. I am just looking for confirmation that this is true. As my SQL brain cannot wrap my head around how VMware would be able to do performance equivalent to RAW LUNs with VMDK files.

Thanks so much for your response and let me know if I provided enough data.
Andrew Hancock (VMware vExpert / EE Fellow)VMware and Virtualization ConsultantCommented:
Okay, just wanted to check we were discussing the same technology.

So for years in VMware vSphere (ESX and ESXi) we were encouraged to use RDMs for Performance.....

and then announcements were made, do not use RDMs for performance VMDKs are fine!

RDM versus VMDK performance (this was reported in 2011!)

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

As for your situation... this whitepaper/guide/Best Practice was produced in August 2018, and should be now regarded as the BIBLE ON VMware vSphere!


This document discusses Partition Alignment, VMFS and Virtual Machine Disks for performance, it does not mention RDMs! Discusses using different VMDKs, and the use of the PV controller!

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sqlagent007Author Commented:
Thank you so much!!!! This is amazing!
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