Subject: Re: partition bookkeeping
To: Bill Studenmund <>
From: Matthew Jacob <>
List: tech-kern
Date: 09/22/1999 14:55:54
> But to do somethign like c0d0l0p0, we have to reserve enough space for the
> maximum values of fields. We IMMEDIATLY run out of room. For instace, the
> SCSI-3 spec permits 16 bits of LUN per disk. So right off the bat we
> can't encode all the possible LUN's on ONE disk!

Bill- what I meant was that in SCSI-3, with SCC hierarchical luns, you get

	Level 1	16 Bits
		Level 2 16 Bits
			Level 3 16 Bits
				Level 4 16 Bits

The current 'SCCLUN' Qlogic Fibre Channel F/W implements the first level
(16 bits). I've tested it for up to 256 luns.

> Say we restrict ourselves to the 8 LUN's of SCSI-2. That's three bits.
> Well, we also want to support fibre channel. A fibre channel bus can be
> approximated as a scsibus with 126 possible targets. To hard code to cover
> them all, that's 7 bits. We started with 12 bits to specify the disks, and
> we've used 10. That leaves us 2 bits to specify the controller.

Actually, even for currently shipping RAID controllers that are nominally
SCSI-2, the target function bits in the IDENTIFY messages are also used,
so in fact, for current parallel SCSI, 32 luns are now not uncommon.