Re: iSCSI: LUs ownership/iSCSI MIB

["Jim Hafner" <hafner@almaden.ibm.com>]

Pierre,

I have some comments on your note with respect to SAM/SCSI.

1) The interpretation depends a lot on what one means by "target".  The
default notion of this term is Target Port (not device, though that is not
necessarily well captured by the MIB).   Clearly SAM-2 allows multiple
target ports to be i/o paths to a common logical unit.
2) the "asymmetric port" proposal from Ken Moe of Sun in T10 (for SPC-3) is
designed to deal with SCSI's view of this active/passive picture you
describe.
3) I don't believe I've ever seen anything in SAM that precludes even
multiple Target Devices from sharing a logical unit.  I believe the
architecture is silent on this, which means that it's OK.
4) Also what's "viewable" by the MIB interface may NOT be the same as
viewable by Report LUNS.  The first is a management interface to get a view
of the whole target device.  The other is a specific host's (more
precisely, initiator's) view of the accessible logical units on a
particular port.  So, the MIB could easily present the same answer on all
ports *of the same target device* and a different target device may present
a different answer.

All this confusion about multiple pathing, etc. is one reason for logical
units to have unique identifiers (EVPD page83h) AND (in my opinion) to keep
most of this discussion out of iSCSI MIB.

Finally, some of us in the N&DT team are wrestling with the mapping of SAM
constructs to iSCSI constructs.  The resolution of that may clarify the
ambiguity of the term "target" in the MIB.

Jim Hafner


Pierre Labat <pierre_labat@hp.com>@ece.cmu.edu on 03-23-2001 10:34:56 AM

Sent by:  owner-ips@ece.cmu.edu


To:   ips@ece.cmu.edu
cc:
Subject:  iSCSI: LUs ownership/iSCSI MIB



Mark (bakke),


Where
=====
In the MIB draft, in the chapter

5.1.  Overall MIB structure


There is:

"Therefore, LUs are "owned" by Targets, and LUNs are owned
   by Sessions."

Problem
======

If a LU is owned by only one target how do you deal with an
active-passive
configuration such as:
active link: portal IP1, LUN=1
passive: portal IP2, LUN=2 (but it is the same LU as the one accessed
through the active link)

As the various portals of a target provide the same LUN view, IP1 and
IP2 can NOT
be two portals of the same target.

In this case we would need two target names, one for the active link and
one for the passive.

But if a LU can be owned by only one target we can't do it.
I think SAM doesn't allows a LU to be shared by two targets and that
seems
to be reflected in the chapter "5.1.  Overall MIB structure".

Solution
======
In the case of an active-passive configuration, the target must
guarantee the same
view on the two paths (active and passive).
Is it a problem for whose who build such targets?


Regards,

Pierre