Re: iscsi : changes involving tgt portal group tag.

[Santosh Rao <santoshr@cup.hp.com>]

Jim,

I agree that a "complete re-configuration" of a target port can result
in a new port name & port identifier. However, the tricky part is the
definition of a "complete re-configuration of an iscsi target port", due
to the concepts of portal groups involving multiple network portals.

For example, if a portal group (aka, an iscsi target port) were to be
re-configured to include a new network portal [moved from another portal
group], then, the target port itself has not changed, since it is still
accessible through its previously used network portals. What has changed
is the individual network portal that has moved from 1 target port to
another. 

Hence, it is sufficient, in this case, to maintain persistence of the
target port name/identifier, without requiring any change in port
name/identifier. By requiring initiators to send the intended TPGT (scsi
target port name/identifier) along with the login request, this allows
the target port to detect that the network portal is being accessed to
connect to a different target port and it can reject the login.

It may be helpful to call out the specific case when a port
name/identifier MUST change. How about something like :

"If a portal group is re-configured such that all its previously
advertised network portals are no longer a part of the portal group,
then, the portal group tag (and thereby, the port name/identifier)
*MUST* be changed to indicate a new target port."

This would allow access to the target port through its un-altered
network portals to continue un-disrupted. More comments inline, in
response to some of your queries.

Thanks,
Santosh

NOTE : In this discussion target port and target portal group are used
to imply the same entity, within a target node.


Jim Hafner wrote:

> SAM-2 requires scsi port names to be persistent and world-wide-unique.
> (SAM-2 Rev 22 Section 4.7.7). Quoting from SAM-2 :
> 
> "A scsi port name shall never change and may be used to persistently
> identify a scsi initiator port or target port...".
> 
> <JLH>
> There are different ways that one can interpret this "persistent" rule. The
> intent was that names shouldn't change over time when *identifying the same
> object*.   What that means is that if the object changes (in any
> discernable way), then the name should change.  So, the object can move to
> a different piece of hardware, but it can still be named the same way.  If
> the object changes, e.g., in this case, reconfigures as a different set of
> network portals with different addressing elements, then I would think that
> the name should change as well.   That's all the persistence one really
> needs.
> 
> I'm not sure what that implies about your recommendation.  I don't see any
> problem with it, either.
> </JLH>

I think we may be in agreement. (See reasoning above). 


> 
> The rationale for (2) is :
> --------------------------
> Consider an initiator node establishing multiple sessions to a scsi tgt
> port, with each session established to a subset of the network portals
> within the tgt portal group.
> 
> Consider an iscsi transport event involving the re-configuration of
> target portal groups on the iscsi target node. This may be preceeded by
> the iscsi sessions seeing an async message "target requests logout",
> followed by session logout, portal group re-configuration, and then, the
> initiator re-establishes sessions.
> 
> Across a transport event that results in such session termination and
> re-establishment, the initiator needs to authenticate that it is still
> speaking to the same [i]scsi target port, in order to ensure that any
> open/active I-T-L nexus traffic on that session is not incorrectly
> routed to a wrong LUN after such a transport event.

> <JLH>
> Under these events, shouldn't all "open/active I_T_L traffic" have been
> aborted, closed or otherwise ended?  So this shouldn't be an issue at all.

On a session logout & re-login, it is not efficient/necessary to close
and re-open each LUN behind that target port, due to the fact that a
target port may have hundred's of LUNs behind it. 

All outstanding I/Os need to be aborted. Once the session is
re-established [and the target port is authenticated to be the same],
further I/O traffic can be resumed without requiring the SCSI ULP to
close and re-open each LUN. Some transport specific clearing effects may
have occurred, which may require additional LUN level activity, in some
cases. 

(There are analogies to the above model in FCP & SRP, which authenticate
port name/identifier on login, allowing I/O resumption after session
termination and re-establishment.)


> To prevent such authentication issues, iscsi can send the iscsi target
> port identifier (portal group tag) explicitly in the login request, and
> the login can be rejected by the target on a portal group tag mis-match.
> (if the network portal does not belong to the addressed portal group
> tag).
> <JLH>
> Did you mean for the initiator to send this TPGT?
> </JLH>

Yes. The initiator login request must include the target portal group
tag, thus identifying the target port to which a session is being
established. 

Login currently carries no addressing information, since the addressing
is implicit, based on the TCP connection in use. The problem with this
approach is that the login implicitly addresses a network portal, and
not the target port. As seen in the earlier example, the association of
network portal <-> target port can change, and such changes can be
detected, if the initiator were to explicitly identify the target port
being logged into.

-- 
##################################
Santosh Rao
Software Design Engineer,
HP-UX iSCSI Driver Team,
Hewlett Packard, Cupertino.
email : santoshr@cup.hp.com
Phone : 408-447-3751
##################################