I had a power issue with my VMware lab, and after that I was unable to see my previous attached LUNs. At the end, it wasn't a VMware issue.
The strange thing with this issue is that I could see the “Devices” but not the “Datastores”, so the storage array should be OK, the networking/paths should be also OK, - it must be an issue in the ESXi - I thought. I was wrong.
I came through these VMware KB trying to reattach the datastore, without success:
More often than not, this problem is to be solved with these KBs (and some extra googling), so check them out first.
Somehow, I keep having random and particular issues with simple solutions but, not that easy to troubleshoot if you don't know where to look at. 😕
After the power outage, only one of my switches, lost its configuration and the MTU setting changed from 9,000 (jumbo frames) to 1,500. The storage array and the ESXi iSCSI adapter all have jumbo frames configured, so the communication was faulty.
After changing back the MTU to 9,000 and rebooting the host, datastores were attached automatically and keep working normally.