Hi
This is meant as a warning to other VMware users, before they run into the same kind of problems, that we did when upgrading to vSphere 6.
We have been running a setup like you see on the picture below for a long time, without any problems. The setup is build up on three DELL R620 servers and two Synology RackStations - One RS3412Pxs and one RS3614RPxs.
But after we upgraded from vSphere 5.5U2 to vSphere 6.0 our SANs and LUNs started crashing. We even experienced 2 disks that died during the 14 days we have been fighting the problem. Both Synos had high CPU usage, high memory load and very often did not respond on neither web interface or SSH. Two times during the past 14 days, the LUNs crashed so hard, we had to make a dissaster recovery.
Just this night, we finally found the root cause: Subnet-A and Subnet-B was in the same broadcast domain (the same IP subnet). Even though I know this is not Best Practices, this have never caused problems, when running vSphere 5.0 and 5.5.
I am NOT saying, that this is a bug in neither vSphere or DSM, only that you might want to make sure, that your iSCSI MPIO setup is using separate VLANs or broadcast domains.
Sorry if this is not the right place for such a warning. I just had an urge to write about it. Please feel free to share is information on any media :-)
The best of luck,
Ernst Mikkelsen (VCP5)
Trifork A/S