Careful preparation is a key element to success. If you restart a storage controller, or even the whole storage, you should be very sure that all ESXi hosts have enough paths to every datstore. Sure, you can use the VMware vSphere C# client or the Web Client to check every host and every datastore. But if you have a large cluster with a dozen datastores and some Raw Device Mappings (RDMs), this can take a looooong time. Checking the path state of each LUN is a task, which can be perfectly automated. Get a list of all hosts, loop through every host and every LUN, output a list of all hosts with all LUNs and all paths for each LUN. Sounds easy, right?
For a long time, I used this PowerCLI script for checking the LUN path state. But now I decided to give something back and I tweaked it a bit for my needs.
Feel free to use and/ or modify it.