Can one LU be in multiple devices?

George Ericson GEricson at TownISP.com
Tue Sep 28 19:44:07 PDT 2004


* From the T10 Reflector (t10 at t10.org), posted by:
* "George Ericson" <GEricson at TownISP.com>
*
Suggest you also look at LU Mapping and Masking models in SNIA SMI-S
specification.  Not as definitive source, but as a source of hard won
consensus across a broad cross section of the storage industry on this
issue.

I weigh in strongly on an LU centric model, where an LU can show up across
multiple ports or targets.  This seems to be common practice, and the words
in SAM-2 and SAM-3 do not rule this interpretation out.  Some of the words,
(e.g. "One of the logical units within the SCSI target device shall be
accessed using the logical unit number zero."), could be interpreted as
containment, however the spec is not otherwise specific about this subject
and others have chosen to interpret the relationship between target device
and LU more loosely.  Especially since until more recently, there has been
considerable confusion about the relationship between target device and
target port.  

BTW, this view is not inconsistent with Target Reset.  Interpret the target
response to a Target Reset as forwarding an LU Reset to all LUs under the
target, then the behavior of the LU with respect to initiators is consistent
regardless of multiple ports or target devices.  In all cases, the LU is
responsible for returning correct status to a new initiator.

Bottom line is I'd like to see SAM-3 more clearly define these
relationships.  Do I dare suggest UML?

George

-----Original Message-----
From: owner-t10 at t10.org [mailto:owner-t10 at t10.org] On Behalf Of Mallikarjun
C.
Sent: Monday, September 27, 2004 8:54 PM
To: T10 Reflector
Cc: Julian Satran; KRUEGER,MARJORIE (HP-Roseville,ex1); Reuter, Jim
Subject: Can one LU be in multiple devices?

* From the T10 Reflector (t10 at t10.org), posted by:
* "Mallikarjun C." <cbm at rose.hp.com>
*
iSCSI allows one "network entity" to host multiple SCSI devices. Since 
all the LU inventory is accesible in the same physical box, each LU can 
actually be a member of multiple SCSI devices and there are some 
advantages in doing that.  A similar situation exists for 
multi-networked devices and virtualization boxes.

I'd like to solicit input from this list on this question: is it 
SAM-3-compliant to have one LU appear in multiple devices?

Jim Hafner and I had an offline email discussion on this.  Jim's 
position was that LUs appearing in multiple devices is legal, because 
(paraphrasing him; Jim, please add/correct):
- It is technically no different from dual-controller arrays (same LU 
can be presented on each I_T nexus, and the nexus ID doesn't include the 
device identifier).
- Storage virtualizers do this today as well.
- Target Reset task management function leads to different results if 
the LU is contained in two devices versus one device, but the recent 
obsolescence of Target Reset helps.

I tended to concur with him, but after looking through the SAM-3 
drawings, I am not sure.  Here's my reasoning:

1. SAM-3's hierarchy diagrams show containment relationships (3.6.1 
makes this clear).

2. Figure 11 and Figure 14 (in 4.7.2 & 4.8 respectively) together imply 
to me that a device server is contained within a SCSI device.

3. From the same figures, the containment relationship between the 
target device object & the device server object is 1-to-1.  IOW, Figure 
11 does not show a Logical Unit being contained in more than one SCSI 
target device (more precisely, the SCSI target device box is not shaded).

Putting these things together, having one LU (i.e. with all the 
composition shown in Figure 14) show up as a contained object in 
multiple discrete SCSI target device instances seems disallowed.

Now the questions are:  Is my reading of the drawings correct?  If so, 
is this prohibition (that each LU belongs to one and only SCSI device) 
intended?

Comments would help.  Thanks.
-- 
Mallikarjun

Mallikarjun Chadalapaka
Networked Storage Architecture
Network Storage Solutions
Hewlett-Packard MS 5668
Roseville CA 95747
cbm [at] rose.hp.com




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