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Date:      Wed, 11 Dec 2013 00:57:42 +0100
From:      Douglas Gilbert <>
To:        Nathan Whitehorn <>,  FreeBSD SCSI <>
Cc:        Hannes Reinecke <>
Subject:   Re: [CAM] Widening lun_id_t to 64-bits
Message-ID:  <>
In-Reply-To: <>
References:  <> <>

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On 13-12-11 12:41 AM, Douglas Gilbert wrote:
> On 13-12-10 10:09 PM, Nathan Whitehorn wrote:
>> Modern SCSI hardware often uses 64-bit logical units (LUNs). The patch found at
>> widens the type of lun_id_t to
>> 64 bits, bumps CAM_VERSION, and begins exposing these to drivers that are marked
>> as supporting extended LUNs. No behavior is changed except that peripheral with
>> very long LUNs that didn't work before will start working. Binary compatibility
>> with old code is also kept. There is, however, a chance that some 3rd party
>> software might be unhappy about the type widening, so I'd appreciate any testing
>> results. Barring any issues, I will commit this on Friday.
> Interesting, Hannes Reinecke is trying to do something
> very similar in the Linux SCSI subsystem. His patch set
> today will be the third attempt in a year (by my count)
> and he might just get over the top this time. There is
> some support in my sg3_utils package for the way Linux
> is implementing "64 bit LUNs". The sg3_utils package
> also supports FreeBSD so I'm interested in what your
> mapping will be.
> Now, as you are no doubt aware, SCSI ( and specifically
> sam5r15.pdf) does not have 64 bit LUNs, it has 8 byte LUNs in
> SCSI order (i.e. big endian). Given that major architectures
> like i386 and x86_64 are little endian, the mapping between
> a 64 bit integer in native form and an 8 byte SCSI LUN is
> a bit of a puzzle. That becomes a little harder when you try
> for low numbered integers representing the T10 3 bit LUNs
> (showing my age), 8 bit LUNs and 16 bit LUNs.
> Down to brass tacks: what exactly will a SCSI REPORT LUNS
> WELL KNOWN LOGICAL UNIT number [T10 (in hex): c1 01 00 00
> 00 00 00 00] be in one of your 64 bits LUNs? Will that be
> the same in little endian and big endian architectures?
> There is also the representation of that LUN in logs; for
> example lun=13907397124296802304 is not very intuitive.
> More examples would be great, perhaps from the 4, 6 and 8 byte
> "extended logical unit addressing format".
> Robert Elliott who has been a T10 technical editor has written
> a paper on this subject but google fails me, perhaps someone
> else can supply the url. His advice was too late for Linux
> and perhaps it is already too late for FreeBSD.

The document I was trying to find was a proposal:
06-003r1, see its overview for a rationale. The Logical Unit
Representation Format section was accepted and can be found
in sam5r15.pdf section 4.7.2

Doug Gilbert

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