Re: iSCSI: "conservative reuse" requirement

["Mallikarjun C." <cbm@rose.hp.com>]

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.

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
> > > ---------------------------------------------------
> > >
> > >
> >