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Date:      Wed, 11 Dec 2013 00:41:18 +0100
From:      Douglas Gilbert <dgilbert@interlog.com>
To:        Nathan Whitehorn <nwhitehorn@freebsd.org>,  "freebsd-current@freebsd.org" <freebsd-current@freebsd.org>, FreeBSD SCSI <freebsd-scsi@freebsd.org>
Cc:        Hannes Reinecke <hare@suse.de>
Subject:   Re: [CAM] Widening lun_id_t to 64-bits
Message-ID:  <52A7A69E.3030703@interlog.com>
In-Reply-To: <52A7830B.2090803@freebsd.org>
References:  <52A7830B.2090803@freebsd.org>

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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.

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




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