Re: iSCSI: "conservative reuse" requirement

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

> 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
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