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How targets/LUNs are assigned

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Quin Hammes
Valued Contributor

How targets/LUNs are assigned

On a XP512 when you assign LUN 00-07 to a HPUX system, its going to be Target 0 / LUN 0-7. 08-0f would be Target 1/ LUN 0-7 etc...

Where does this translation occur? Is that part of the driver for the HPUX box or does that happen on the XP?

I ask this because we are starting to work with Windows systems and they handle the target/LUN differently. (the target is assigned by the order in which it finds storage ports and then the LUNs on the XP come in as the same LUN in NT (XP LUN 09 would be LUN 09 on NT also).

Points for ALL!

Eugeny Brychkov
Honored Contributor

Re: How targets/LUNs are assigned

Answer: this is done at driver level in HPUX.
Let's go to the history of HPUX addressing. Not so long time ago when there was only SCSI standard implemented widely and it had its limitations (by cable lenght, device number etc) it was made that the last 3 digits of device address represent bus (0..15), target (0..15) and LUN (0..7) so there was a limitation: every target (device having "target" SCSI ID) can have only 7 LUNs and one bus can have only 16 (including SCSI adapter) targets, and there could be only 16 buses. When FC protocol was introduced decision was made to use the same scheme to represent FC LUNs, and it was decided to use the following formula:
FC AL_PA=target*8+LUN
so if you have AL_PA value you can build it from only one combination of (target,LUN).

It's brief explanation, and remainder of device addess depends on which port type do you have: private loop (hubs/direct connect) or fabric (direct fabric attach).
Hope it helps.