RE: iSCSI: ISIDs

[Black_David@emc.com]

Rod,

> 	I don't see how having the target assign the ISID, and effectively
> the whole SSID helps with the original problem of recovery from an
> uncontrolled restart. In fact, I think it makes it worse.

It helps with the following problem:

	When the initiating adapter/interface assigns the ISID, mistakes
	in assigning the ISID (inadvertently using the same ISID twice
	on two different interfaces) can cause sessions to step on each
	other when that was not the intended result.  If there's no
	ISID, and TSID is dynamically assigned by the Target, this
	problem can't happen.

Recovery from an uncontrolled restart wouldn't change in principle.
In order to recover session resources (e.g., persistent reserves), the
HBA had to remember the ISID - now it would have to remember the TSID.
In practice, this may be an important difference - an HBA can "remember"
the ISID(s) by always using the same ISID value(s), whereas the TSID(s)
have to be stored after being provided by the target.

> 	Without and initiator assigned ISID the target is presented with no
> information with which to distinguish HBAs in a host. Assuming that
> HBA 1 has an existing session, how can the target tell the difference
> between HBA 2 sending a leading login with the intent of starting a
> new session, and HBA 1 sending a leading login after an uncontrolled
> restart?

HBA 2 sends a TSID of zero, HBA 1 sends the TSID of the session it's
trying to recover.  Two HBAs are not necessary to create this scenario -
it can occur with two sessions on a single HBA.  If the HBA has forgotten
the identity of the session it's trying to recover, that session is
unrecoverable, but see above comment on differences in remembering
an ISID vs. a TSID.

Thanks,
--David
---------------------------------------------------
David L. Black, Senior Technologist
EMC Corporation, 42 South St., Hopkinton, MA  01748
+1 (508) 435-1000 x75140     FAX: +1 (508) 497-8500
black_david@emc.com       Mobile: +1 (978) 394-7754
---------------------------------------------------

> -----Original Message-----
> From: Rod Harrison [mailto:rod.harrison@windriver.com]
> Sent: Friday, September 07, 2001 5:34 AM
> To: Black_David@emc.com; ips@ece.cmu.edu
> Subject: RE: iSCSI: ISIDs
> 
> 
> 
> 	I don't see how having the target assign the ISID, and effectively
> the whole SSID helps with the original problem of recovery from an
> uncontrolled restart. In fact, I think it makes it worse.
> 
> 	Without and initiator assigned ISID the target is 
> presented with no
> information with which to distinguish HBAs in a host. Assuming that
> HBA 1 has an existing session, how can the target tell the difference
> between HBA 2 sending a leading login with the intent of starting a
> new session, and HBA 1 sending a leading login after an uncontrolled
> restart?
> 
> 	- Rod
> 
> -----Original Message-----
> From: owner-ips@ece.cmu.edu [mailto:owner-ips@ece.cmu.edu]On Behalf Of
> Black_David@emc.com
> Sent: Thursday, September 06, 2001 10:40 PM
> To: ips@ece.cmu.edu
> Subject: iSCSI: ISIDs
> 
> 
> Time to back up and regroup on this topic.  It's clear that ISID
> management needs more attention, and hence there are some issues
> more basic than the attempted consensus call to work out.  So,
> I'm going to abandon the attempted consensus call, and try to
> make progress on the underlying ISID issue.  Those whose posts
> have called ISID management into question should not feel it
> necessary to apologize - this is an important issue to unscramble.
> 
> A rough summary of where we find ourselves is:
> - iSCSI Initiator Names span network interfaces/adapters,
> 	as do iSCSI Target Names.
> - Multiple sessions are allowed between an iSCSI Initiator
> 	and Target.  These are identified by an ISID and a
> 	TSID.
> - The ISID is the Initiator portion of the Session identifier
> 	and is used to track certain resources associated with
> 	the session (e.g., persistent reserves).
> - The TSID is the Target portion of the Session identifier,
> 	and serves primarily to make sure that all session
> 	identifiers (SSID = ISID||TSID) are unique at the
> 	Target.
> 
> Michael Schoberg suggest getting rid of the ISID:
> 
> > ISID - initiator-defined session-identifier
> >
> > Since initiators don't know about other initiators connected to this
> target,
> > this has the potential of causing problems.  Eliminate it.  The
> initiator
> > should simply state it's intentions of establishing a new session or
> adding
> > a connection to an existing session (via binary flags).
> 
> The implication of this would be to make SSID = TSID, dynamically
> assigned by the Target (0 is a reserved value on Login asking Target
> to do this assignment), and partitioned appropriately across
> interfaces.
> Recoverable session resources would be associated with the combination
> of <iSCSI Initiator Name, TSID> at the iSCSI Target, and the initiator
> tracks via <iSCSI Target Name, TSID> - in both cases the TSID replaces
> the ISID.  From a functional standpoint, this looks like it works,
> and has the nice additional property that session resources can be
> recovered through any iSCSI Initiator interface vs. having to go back
> to the one that's allowed to use the ISID for that session if ISIDs
> are statically partitioned.
> 
> Does this cause any problems for the SAM-2 to iSCSI mapping?
> 
> Thanks,
> --David
> 
> ---------------------------------------------------
> David L. Black, Senior Technologist
> EMC Corporation, 42 South St., Hopkinton, MA  01748
> +1 (508) 435-1000 x75140     FAX: +1 (508) 497-8500
> black_david@emc.com       Mobile: +1 (978) 394-7754
> ---------------------------------------------------
>