[dm-devel] EMC Clariion ghost devices

[Gaál Norbert]

Hi!

>> Is that a LUN 0 backed by actual storage or a LUN_Z?

>	I don't quite know what you mean by LUN_Z.  My EMC guy says it
>is a "ghost" projected because the second SP owns no disks, but SCSI
>requires a LUN 0 to be scannable.
>
>Joel

If no LUN 0 is present (or no LUN is in the Storage Group), and arraycommpath is enabled the Clariion 'emulates' a LUN this is a LUNZ.

>From a EMC primus article:

A SCSI-3 (SCC-2) term defined as "the logical unit number that an application client uses to communicate with, configure and determine information about an SCSI storage array and the logical units attached to it. The LUN_Z value shall be zero." In the CLARiiON context, LUNz refers to a fake logical unit zero presented to the host to provide a path for host software to send configuration commands to the array when no physical logical unit zero is available to the host. When Access Logix is used on a CLARiiON array, an agent runs on the host and communicates with the storage system through either LUNz or a storage device. On a CLARiiON array, the LUNZ device is replaced when a valid LUN is assigned to the HLU LUN0 by the Storage Group. The agent then communicates through the storage device. The user will continue, however, to see DGC LUNz in the Device Manager. 
LUNz has been implemented on CLARiiON arrays to make arrays visible to the host OS and PowerPath when no LUNs are bound on that array.  When using a direct connect configuration, and there is no Navisphere Management station to talk directly to the array over IP, the LUNZ can be used as a pathway for Navisphere CLI to send Bind commands to the array. 

LUNz also makes arrays visible to the host OS and PowerPath when the host·s initiators have not yet ·logged in to the Storage Group created for the host.  Without LUNz, there would be no device on the host for Navisphere Agent to push the initiator record through to the array. This is mandatory for the host to log in to the Storage Group. Once this initiator push is done, the host will be displayed as an available host to add to the Storage Group in Navisphere Manager (Navisphere Express).

LUNz should disappear once a LUN zero is bound, or when Storage Group access has been attained.

To turn on the LUNz behavior on CLARiiON arrays, you must configure the "arraycommpath." 

Regards:
 Norbert Gaal