RE: iSCSI: ISIDs

[Black_David@emc.com]

One more whack at this ...

> The major difference is "control". In the current scheme the initiator and
> target could independently allocate and reuse their parts.
> 
> The target allocates only scheme leaves this to the target (it has to do
> garbage collection) under the assumption that the target (unlike the
> initiator) has a central point of control.

Yes, I'm assuming that targets will have a single vendor in charge
of all the software on the target and making it work right if there are
multiple HBAs; I don't see any corresponding vendor who's in a position
to do the same for the initiator ...

> Aren't targets going to have several HBA and have to carefully allocate
> their SSIDs?

But they'll have one vendor in charge of making this work.  In contrast,
for an initiator with HBAs from two different vendors, both of those
vendors and the vendor of the OS platform have to be involved, and I
don't see any of them taking full responsibility in the early going.
I'm worried we risk heading the iSCSI Initiator Name down the same
path as Fibre Channel, where the FC Node WWN was supposed to be associated
with the server into which the HBAs were plugged, but got associated
with the HBA because there was no way to coordinate across HBAs.
If anything ever goes wrong with ISIDs (and things will), the fastest
way to fix it will be to require each HBA to have its own iSCSI Initiator
Name, independent of the intent of the iSCSI naming architecture.

> In the event an initiator wants to rebuild a session does he have to
> connect to the same HBA and thus a failover to another HBA 
> can't be done.

No, this is controlled by portal groups.  To failover across two HBAs
on the target, they have to be in the same portal group, and the target
is responsible for coordinating TSID usage within each portal group.
If the target can't coordinate TSID usage, it puts the HBAs into different
portal groups and gives up this failover ability as a consequence, and
that's the case independent of whether ISIDs exist or not.

> For all the above no to happen we have to add some management somewhere
and
> it looks to me that the current scheme is simpler than a target centric
one
> as it keeps the allocation and release of numbers at the same end.

I'm sorry, that doesn't parse for me.  It looks like the same sort of
mechanism is being put into both initiators and targets for symmetry,
without considering whether two instances of the mechanism are actually
necessary.  In a target-centric scheme, the target is doing the "allocation
and release" of numbers.  The target does have to be careful about not
aggressively reusing TSID values, when there's potentially recoverable
state around (or the initiators may think that there's state to be
recovered even though the target's lost it all in a reboot), but recall
that aggressive reuse of ISIDs is what created the Option A/B/C mess
(and not aggressively reusing ISIDs would make that easier to solve).
I don't understand how two instances of similar mechanisms is "simpler"
than one.

--David

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David L. Black, Senior Technologist
EMC Corporation, 42 South St., Hopkinton, MA  01748
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