Newly presented LUNs are not visible on Esxi host

Today when I was migrating two MSCS cluster VMs from one host to another (cold migration), found 5 LUNs missing on target host so asked the Storage admin to present these LUNs on target host. Once the Storage admin confirmed the same, we re-scanned the Storage/HBA adapters for datastore/LUNS, after the rescan when checked for the newly added LUNs, was surprised to see on one host only two LUNs were visible while on another host all five LUNs were visible. Then I checked with storage team, they confirmed that everything is fine from their end and I might need to restart the host to make the LUNs visible.

After evacuating all the VMs, rebooted the host but even after reboot LUNs were unavailable. Later when I further investigated, the issue turned out to be related to max storage path, yes this host was already having the max storage paths that is why newly mapped LUNs were not visible.

Note: Local storage, including CD-ROMs, are counted in your total paths.

You can see how many paths are being used on a specific host by Selecting the host, going to Configuration => Storage Adapters => Storage Adapter

As most of us would be aware, the VMware vSphere Host storage path limit as of vSphere 5.x is 1024 and the maximum LUNs per host is 256  (refer to configuration maximums) and as this host was already having the max supported paths (552+471+1= 1024), thus was unable to add new LUNs/Paths. 

Fix: To fix this issue, ask the storage team to reduce the number of paths per LUN so that there are fewer than the 1024 total limit, or reduce the number of LUNs presented to the host.

Note: put the host in maintenance mode during the storage path correction and once done, re-scan the host/storage adapters for datastore/LUNs.

Reference: David Ring's blog post on host path limit, VMware KB#1020654.

That’s it... :)