Re: iSCSI: "conservative reuse" requirement

["John Hufferd" <hufferd@us.ibm.com>]

Mallikarjun,
Comments in line between [Huff/] and [\Huff].

.
.
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John L. Hufferd
Senior Technical Staff Member (STSM)
IBM/SSG San Jose Ca
Main Office (408) 256-0403, Tie: 276-0403,  eFax: (408) 904-4688
Home Office (408) 997-6136, Cell: (408) 499-9702
Internet address: hufferd@us.ibm.com


"Mallikarjun C." <cbm@rose.hp.com>@ece.cmu.edu on 01/02/2002 01:21:13 PM

Please respond to cbm@rose.hp.com

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


To:    Black_David@emc.com
cc:    ips@ece.cmu.edu
Subject:    Re: iSCSI: "conservative reuse" requirement



Now that I'm back from vacation, let me offer some (delayed)
comments on this.  I also read Jim's reply where he appeared
to mostly agree with me.

>...as long as it presented
> each of them to all target ports...

I disagree with your assumption that an initiator must
"present" *all* its ports to every target port in a target
device.  Each distinct initiator port (signified by a unique
ISID) may want to talk to only one target port (i.e. one portal
group) - that's a typical usage of multi-HBA (i.e. multi-port)
hosts talking to multi-FC port targets today.  As far as I
can understand, a mandatory "conservative reuse" would
preclude this usage.  Please comment if your visualization
of conservative reuse is different.

[Huff/]
We have now fallen into the problem that Jim, Julian, Marjorie, and I spent
the Holidays exchanging e-Mails about.  You use the term Initiator to
represent the Total Initiator Node (and indeed our draft definitions
encourage that view).  However, some others, when they talk, use the SCSI
interpretation of the word Initiator, which is the SCSI Initiator Port.  So
as Marjorie has clearly pointed out to me, I need to be more precise, and I
think the rest of us also.  We need to fully qualify the term Initiator
when we use it, in these types of discussions.  We need to say iSCSI
Initiator Node, or SCSI Initiator Port etc.  So let me try that below, in
hopes of perhaps clearing up some concepts.

The use of Conservative Reuse applies to the how the Name of the SCSI
Initiator Port will be chosen.  The Name of the SCSI Initiator Port is made
up of the iSCSI Initiator Node Name and the ISID.

We are attempting to ensure that any one SCSI Initiator Port will not pick
various different ISIDs in a manner that will prevent the Persistent
Reserves from being reestablished across a Session Restart.  To do that we
strongly suggest that the Vendor follow the Type and Vendor ID approach to
creating and ISID as specified in version 9 of the Draft. Also they should
pick the 16 bit Qualifier part in a way that will usually be the same
number each time a Session from that SCSI Initiator Port is restarted.  The
easiest way to do this is for the iSCSI Process to Identify the SCSI
Initiator Port with the same ISID Qualifier every time an iSCSI Session is
started with any SCSI Target Port (via its iSCSI Target Portal Group).

When this is done, everything works.  This is called Conservative Reuse
since when we start/restart a session we are not choosing a different ISID.

Please note I did not restrict the actions of any other SCSI Initiator Port
within the iSCSI Initiator Node.  They can each have their own ISID and use
the iSCSI Transport to connect to what ever SCSI Target Nodes they wish.
If those other iSCSI Initiators have HBAs which have been made by the same
vendor (and if that vendor does not support Multiple Connections per
Session across the HBAs), it is the Vendors responsibility to chose an ISID
Qualifier for its second (and subsequent) SCSI Initiator Port, which does
not conflict with the first one.  When done correctly, then if by chance,
the second SCSI Initiator Port connects to the same SCSI Target Port to
which the First one connected, they will each be seen, by the Target, as
different Sessions from different SCSI Initiator Ports and things are
completely legal.

In this case, as it is in Fibre Channel, the Host (iSCSI Initiator Node)
will have to be very careful how work is handed out to the different SCSI
Initiator Ports, so that Work arrives in the correct order and there is no
conflicts with Reserves.  (The process that does that balancing between the
HBAs is what, in Fibre Channel, we have called a Wedge Driver).

So what I have said above, is that when we carefully qualify the Term
Initiator, we can see that Conservative Reuse does NOT cause any
restrictions on the way we use the Sessions within an iSCSI Initiator Node.
[\Huff]

OTOH, I can see that the ability of *one* initiator SCSI
port to establish nexii with multiple target SCSI ports in
a target device is highly desirable (perhaps even T10-required
shortly).  IMO, iSCSI had already met this need by defining
and enabling "conservative reuse", and that's where we should
stop.  I would actually advocate wording along the lines of -

    "When a given initiator implementation seeks to present
     a single SCSI port abstraction to multiple target portal
     groups (thus multiple SCSI target ports) of an iSCSI target
     node, conservative reuse is the means to realize it.
     Conservative reuse hence MUST be used in this case."

and leave it at that.
--
Mallikarjun


Mallikarjun Chadalapaka
Networked Storage Architecture
Network Storage Solutions Organization
MS 5668     Hewlett-Packard, Roseville.
cbm@rose.hp.com


Black_David@emc.com wrote:
>
> Not exactly.  "Conservative Reuse" would allow an initiator
> to present multiple initiator ports, as long as it presented
> each of them to all target ports (assuming that the connectivity
> exists).  Why would an Initiator want to present different
> ports to different target portal groups?  I don't think there's
> another example in which SCSI behaves this way in practice.
>
> --David
>
> > -----Original Message-----
> > From: Mallikarjun C. [mailto:cbm@rose.hp.com]
> > Sent: Monday, December 24, 2001 12:47 AM
> > To: Black_David@emc.com
> > Cc: ips@ece.cmu.edu
> > Subject: Re: iSCSI: "conservative reuse" requirement
> >
> >
> > > I think this is headed towards "conservative reuse" being a MUST if
> > > we're serious about support for shared persistent reservations.
> >
> > Mandating "conservative reuse" appears to force initiators to
> > always act
> > as a single initiator port (wrt one target; assuming only one
> > session as an
> > example) per initiator device - which rules out the case of
> > an initiator
> > intentionally wanting to present a different port to each
> > target portal group.
> >
> > IMHO, if iSCSI provides an architected mechanism to support shared
> > persistent reservations ("conservative reuse"),  that should
> > be completely
> > adequate to meet the expectations to be a legal SCSI protocol.
> > --
> > Mallikarjun
> >
> > Mallikarjun Chadalapaka
> > Networked Storage Architecture
> > Network Storage Solutions Organization
> > Hewlett-Packard MS 5668
> > Roseville CA 95747
> >
> > ----- Original Message -----
> > From: <Black_David@emc.com>
> > To: <santoshr@cup.hp.com>
> > Cc: <ips@ece.cmu.edu>
> > Sent: Friday, December 21, 2001 4:50 PM
> > Subject: iSCSI: "conservative reuse" requirement
> >
> >
> > > Santosh Rao writes:
> > >
> > > > I am opposed to the suggestion that "conservative re-use"
> > of ISIDs be
> > > > made a MUST. This is really only required when initiators
> > need to be
> > > > using the new T10 Reservation scheme that can be shared
> > > > across initiator ports.
> > >
> > > Those reservations are a Target feature.  With this
> > approach, the ability
> > > to use the target feature depends on details of the initiator
> > > implementation.
> > > More below ...
> > >
> > > > For those initiators that don't care about this type of
> > reservation,
> > > > conservative re-use is of no use and initiators may like to assign
> > > > ISID's in a per-initiator node fashion, thereby, being
> > able to use these
> > > > ISIDs as a lookup index for the sessions on that initiator node.
> > > >
> > > > Hence, I suggest that "conservative re-use" be worded as
> > > > "encouraged to use" or something to that effect, but not MUST USE.
> > > >
> > > > Comments ?
> > >
> > > The "initiator" is more than one entity.  The iSCSI HBA/NIC
> > and driver
> > > doesn't know whether shared persistent reservations are
> > being used and
> > > shouldn't have to care - they're just more SCSI commands to
> > transport.
> > > Some other entity (e.g., clustering software) will be generating the
> > > shared persistent reservations.  This raise the possible scenario
> > > involving a target that supports the new shared persistent
> > reservations
> > > and an entity that wants to use them.  The entity detects
> > (via SCSI means,
> > > e.g., something in a mode page) that the Target supports
> > shared persistent
> > > reservations, tries to use them, only to discover that they
> > don't work
> > > because the iSCSI HBA/NIC doesn't implement "conservative reuse".
> > >
> > > I'm worried about this causing both interoperability issues
> > and possible
> > > T10 issues -- from a T10 viewpoint, if shared persistent
> > reservations
> > > don't work, the initiating entity should have some SCSI-level means
> > > of determining this ... if that means exists only on the Target, the
> > > above scenario is iSCSI's problem (Target can't query Initiator to
> > > determine whether it does "conservative reuse"), and having
> > a separate
> > > initiator side means that the entity has to check only for
> > iSCSI (and
> > > not for any other SCSI transport) does not seem like the right
> > > approach.
> > >
> > > I think this is headed towards "conservative reuse" being a MUST if
> > > we're serious about support for shared persistent reservations.
> > >
> > > Comments?
> > > --David
> > > ---------------------------------------------------
> > > David L. Black, Senior Technologist
> > > EMC Corporation, 42 South St., Hopkinton, MA  01748
> > > +1 (508) 249-6449 *NEW*      FAX: +1 (508) 497-8500
> > > black_david@emc.com         Cell: +1 (978) 394-7754
> > > ---------------------------------------------------
> > >
> > >
> >