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Date:      Tue, 10 Dec 2013 17:57:22 -0600
From:      Nathan Whitehorn <>
To:,  "" <>, 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 12/10/13 17:41, 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".

We're following the path-of-least-resistance from Solaris. I've 
momentarily forgotten how this works in the Linux patches, but the 
approach is as follows (this has actually been in HEAD for a couple 
months now). Extended LUNs are stored in host byte order with swizzled 
16-bit word order so that, for devices implementing LUN addressing (like 
SCSI-2), the numerical representation of the LUN is identical before and 
after the change. Thus this keeps most behavior, and user-facing LUN 
IDs, unchanged. A macro (CAM_EXTLUN_BYTE_SWIZZLE) is provided to 
transform a lun_id_t into a uint64_t ordered for the wire.

Most of the kernel prints these in hex as per SAM5. camcontrol prints 
them out in various ways if it knows the addressing component to which 
they correspond and otherwise prints them in hex. This seemed like by 
far the least painful approach: it has a (fairly) simple direct mapping 
onto the wire format, nothing changes for users, and almost nothing 
changes for code.

> 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.

Hopefully this corresponds to that advice, whatever it was :) Any 
suggestions for changes would be appreciated if you have them.

> Doug Gilbert
> P.S. I know Linux has stupid typedefs in its kernel and
>      was hoping FreeBSD would be better. That was until
>      I saw u_int64_t rather than the standard (and
>      shorter) uint64_t

CAM is old still, so I've tried to keep the existing style. Updates are 
probably a good idea.

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