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Date:      Tue, 10 Dec 2013 17:58:27 -0600
From:      Nathan Whitehorn <nwhitehorn@freebsd.org>
To:        dgilbert@interlog.com, FreeBSD SCSI <freebsd-scsi@freebsd.org>
Cc:        Hannes Reinecke <hare@suse.de>
Subject:   Re: [CAM] Widening lun_id_t to 64-bits
Message-ID:  <52A7AAA3.5050404@freebsd.org>
In-Reply-To: <52A7AA76.3060208@interlog.com>
References:  <52A7830B.2090803@freebsd.org> <52A7A69E.3030703@interlog.com> <52A7AA76.3060208@interlog.com>

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On 12/10/13 17:57, Douglas Gilbert wrote:
> 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
>>> http://people.freebsd.org/~nwhitehorn/lun64.diff 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 (www.t10.org 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 www.t10.org 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
>
>

Ah, OK. That we are following.
-Nathan



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