iSCSI: "conservative reuse" requirement

[Black_David@emc.com]

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
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David L. Black, Senior Technologist
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