https://blogs.msdn.microsoft.com/clustering/2012/04/06/how-to-add-storage-to-clustered-shared-volumes-in-windows-server-2012/        

Shouldn't you be doing just a "Quick Format" of NTFS on the volume first and then add it as a CSV into cluster?

Regards, Michel.

Yup, that's what I'm doing. But even a quick format is slooooow. And I'm seeing it on LUNs presented by VNX, Unity, Unity AFA, XtremIO. Quick Formatting (in diskpart) is always slow. It is slightly faster on all flash arrays, but it's still slow. On average it takes up to 15 minutes for a 2TB LUN, but like in my question, the 9TB LUN took much longer.

That does not sound right. Recommend opening a service request. Run the test, collect an EMCReports and attach to the service request.

Regards, Michel.

I know disabling VAAI on VMware is (or was?) a best practice to speed things up, but there must be another way, right? I will open an SR to have Dell EMC produce a valid answer, but I'd appreciate it if forum peeps have solutions as well :-)

Normally for simple issues yes... but my guess is that there maybe a connection issue, and an EMCReports with the HBAinfo.txt file collected may indicate an issue down 1 x HBA.

every Hyper-V host has the same "problem" and we have dozens of them in various clusters, spread over various data centers, connected to a variety of storage systems (VNX2, Unity, Unity AFA, VMAX, XtremIO, you name it). It simply cannot be a defective HBA or drivers since we have quite a variety of them.

thank you, that was what I was looking for!

Did it help the issue RRR ?

Absolutely! It worked like a charm. Disable before formatting and enabling it right after if how it's done! Formatting now takes seconds instead of (sometimes) hours.