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Auto LUN Discovery on ESXi hosts

Did you know that any newly presented LUNs/paths added to an already discovered target will automatically be discovered by your ESXi host without a rescan of the SAN? In this example, I currently see two iSCSI LUNs from my NetApp array:

Let’s see what happens when I add new devices to my ESXi host from a new target.

My next step is to add a new LUN, but this time from my EMC array (which is a brand new target). Using the CLI on my EMC array, I can add a new LUN (ID 10) as follows to my ESXi host:

Now if I initiate a rescan from my vSphere Client, this new LUN from my EMC array appears. I have to do this manual rescan in order to discover the target for the first time (in this context, target represents a controller port on the EMC storage array).

The EMC array shows up as target 2 (T2) in the RunTime name above. Now I go ahead and present a second LUN (ID 20) from my EMC array to my ESXi host.

Since the target has already been discovered, there is no need to automatically rescan. The new LUN shows up automatically after a few moments on my ESXi host and in my vSphere Client:

I guess the next question is what triggers the rescan. There are in fact two cases which will discover new or removed LUNs or device paths automatically. The first of these is the periodic probe which runs every 5 minutes (tunable) and works with existing targets to discover new paths. The other is an adapter event which is triggered when a new target is found or an old target is removed. We use the SCSI Report LUNs command where possible but will also use SCSI Inquiries. The advanced setting is Disk.DeviceReclaimTime.

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