Persistent Reservation Proposal - Group Reservations

Roger Cummings roger_cummings at
Tue Dec 18 07:03:02 PST 2007

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I'm sorry, I don't think it's as cut and dried as you make out. This
gets into some of the corner cases that I listed in my first response.
The point to be made in response to Fred's case is that a third-party
can create registrations for a downed initiator (via the SPEC_I_PT) bit,
so that when it comes up again it will be able to participate in the
reservation without having to register itself.
Also, you say that "We have made provisions for adding members once the
reservation exists, but only one of the reservation holders can add
another entity." Two things in response to that:
1) I didn't see any specific provision for adding members in your
proposal, so I presume you'd just issue another RESERVE with the same
type and the whole list of transport IDs to be included again, and thus
the Target would have a whole lot of work to do again to set up another
2) I that really what you want, that an member of the existing group can
reissue the RESERVE with a whole bunch of different TransportIDs,
perhaps excluding some that were previously there?
From: owner-t10 at [mailto:owner-t10 at] On Behalf Of Kevin D
Sent: Monday, December 17, 2007 3:54 PM
To: Knight, Frederick
Cc: t10 at; Christine R Knibloe
Subject: RE: Persistent Reservation Proposal - Group Reservations
	This is being proposed for SPC. 
	There are multiple types of reservations.  In an environment
where one node of a cluster must join later, one of the other types can
be used.  Either that or have an existing node in your cluster add the
new node.  The whole intent of this Group reservation is to lock out
everybody that is not explicitly specified during the reserve.	We have
made provisions for adding members once the reservation exists, but only
one of the reservation holders can add another entity.	The new entity
cannot add itself.  This is the whole point of reservations (i.e., lock
out others from doing stuff while I think I have exclusive rights). 
	To put it in other word's, to allow somebody to join the
reservation of their own accord without permission is EXACTLY what I am
trying to protect against. 
	Kevin D. Butt
	SCSI & Fibre Channel Architect, Tape Firmware
	MS 6TYA, 9000 S. Rita Rd., Tucson, AZ 85744
	Tel: 520-799-2869 / 520-799-5280
	Fax: 520-799-2723 (T/L:321)
	Email address: kdbutt at 
"Knight, Frederick" <Frederick.Knight at> 
12/17/2007 01:38 PM 
Kevin D Butt/Tucson/IBM at IBMUS 
RE: Persistent Reservation Proposal - Group Reservations
	Sorry, you can't require everyone to register before the
	That's like saying my whole cluster can't boot because 1 node is
down.  You need 
	to have a way for a "down" initiator to join the fun after the
	I helped write a host cluster product that used a shared tape
(failover model).  The 
	backup application would write to the tape.  If a system failure
ever happened, the 
	backup application would failover to a different host.	It would
skip backwards on 
	the tape for a few records, recognize where it left off, and
then resume operation. 
	BUT, for some protection, we used reservations to make sure only
1 initiator at a 
	time could access the tape.  The interesting point however, is
that we were in the 
	process of upgrading from old SCSI-2 RESERVE to using PR.
Because, we also 
	have multiple HBAs in the host, and we wanted to be able to use
more than 1 of 
	those HBAs (so we needed multiple reservations - aka PR).
Having this idea 
	(group reservations) would have been a real nice addition. 
	As for the RA/AR differences.  It seemed to be timing.
Registrants Only was fairly 
	early on (as I remember), and so implemented by several O/S
vendors.  Later on, 
	some issues were found (which got complicated spec-ees added to
address), but also, 
	the All Registrants was added (which didn't have those issues).
But, since there were 
	implementations, it couldn't be removed like the other old PR
types that no one ever 
	used.  Anyway, I agree, they offer basically the same
capabilities, but RO is already 
	out there, and AR is probably what new implementers are using
(it's easier to understand 
	and implement from the host side).  Most of the differences are
already documented, 
	so there wouldn't be that much extra for you to write to have
both types (which I think 
	would be better than bit somewhere - do it the same way all the
others are done).  But, 
	you could also just do the AR version, and let someone else add
the RO version if they 
	want it. 
	Are you proposing this for tape only? or SPC in general?  I
assume SPC in general. 
	    Fred Knight 
	From: Kevin D Butt [mailto:kdbutt at] 
	Sent: Monday, December 17, 2007 9:51 AM
	To: Roger Cummings
	Cc: t10 at
	Subject: RE: Persistent Reservation Proposal - Group
	Thank you for your feedback.  I am certainly willing to
entertain  other methods for accomplishing the end goal in an easier
fashion.  I am not sure I understand how your proposed method makes it
more backward compatible.  In my proposal PRin would show a different
type of reservation and hence the application clients would not try to
join the reservation because they don't know about the type.  In your
proposal, application clients would not be allowed to register.  This is
a deviation from what they can always do today - unless there is a
resource issue.  This seems more disruptive to me.  I would assume that
there would be a new additional sense code added for UNABLE TO REGISTER
BECAUSE A GROUP RESERVATION IS IN PLACE (or analogous).  This would be a
new thing for failure to register and there would be pain at the
register point.  Perhaps that is better than at the reserve point - but
I would think that it would be better handled as a reservation conflict
since that is what it is instead of something the application client
does not understand. 
	As for "all registrants" type vs. "registrants only" I didn't
see where the difference would be interesting, but I am not opposed to
providing a way to switch between which of these two types is done.
Whether it is additional types or some bit during registration etc. 
	As for some of the corner cases mentioned below, if each I_T
nexus that is supposed to be part of the group reservation is required
to be registered before the reservation is made, and if the reservation
is released when the last group reservation participant is unregistered,
then I think we don't have an issue. 
	I would prefer that we work together to shape a mutually
beneficial proposal as opposed to have "competing" proposals.  I am
willing to modify my proposal where it can be made easier and such.  I
am not sold that my proposed method is the only way or even the best -
it's just the way I thought of doing it.  I admit that I have always
been very confused about the usefulness of RA and AR types.  They make
absolutely no sense in the tape world. 
	Kevin D. Butt
	SCSI & Fibre Channel Architect, Tape Firmware
	MS 6TYA, 9000 S. Rita Rd., Tucson, AZ 85744
	Tel: 520-799-2869 / 520-799-5280
	Fax: 520-799-2723 (T/L:321)
	Email address: kdbutt at 
"Roger Cummings" <roger_cummings at> 
Sent by: owner-t10 at 
12/14/2007 12:10 PM 
Kevin D Butt/Tucson/IBM at IBMUS 
<t10 at> 
RE: Persistent Reservation Proposal - Group Reservations
	First of all, let me say that I completely support what you're
trying to do here. I think that providing a method in persistent
reservations (PRs) to support shared access between ONLY a
specifically-designated set of systems is a worthy goal, and something
we should do in SPC-4. 
	Adding a set of Transport IDs to Reserve as per your document
08-024 & 08-025 is certainly possible, but it's a massive change to the
way that PRs work today, and it throws up a bunch of nasty corner cases
and backwards compatibility issues. 
	The massive change comes from the fact that now the Target will
have to remember which registrations are in the Reservation, and which
are not. It will probably have to preserve all of the transport
information for the life of the reservation. 
	The corner cases are things like, what happens if there's no
longer a registration that corresponds to the transport ID in the
Reserve? Does the Reserve succeed? What happens if a registration comes
in later, after the reservation has been established - does that device
it get access? 
	Backwards compatibility issues may arise like this: An existing
device registers, and finds it has no access, so it does a PR In and
finds out that a reservation is in place, retries its access and still
it has no access. What does it do next, preempt the reservation because
it assumes the Target is broken? 
	Reserve also has to be an "atomic" command, and I've always
thought that was why it's functionality is as compact as it is today.
Most of the complex operations related to addresses and keys are done at
registration time, and those operations don't have to be atomic. 
	One more thing: you chose for your new "group" reservations to
follow the "all registrants" approach is terms of the definition of the
reservation holder. While that's fine by me (obviously), I suspect there
are also situations where group reservations that follow the
"registrants only" approach might be useful. 
	The bottom line from my point of view is this: Your proposal is
feasible and we can probably make it work. But I wonder if there's an
easier way to achieve the same goal that is more compatible with
existing practice and requires less of a change in functionality on the
Target side. 
	What if we didn't add any new reservations types, but instead
added some new functionality to the registration process? What I'm
thinking of a new Register feature that causes the Target to kill all
existing registrations, create the registrations identified in the
transport IDs in the Register command, and not accept any future
registrations. That way, we don't need any changes to Reserve, and an
Initiator with existing functionality would just not be able to register
and therefore would not be confused. 
	Does that make sense to you? Is there a chance this is an easier
approach? If so, I'll write up a detailed proposal that's the equivalent
of 08-025r0 and we can compare and contrast at the next CAP. 
	Again, thanks for getting this started, I think it's a
worthwhile endeavor and I'll be glad to put some cycles towards defining
this sort of functionality for SPC-4. 
	From: owner-t10 at [mailto:owner-t10 at] On Behalf Of
Kevin D Butt
	Sent: Monday, December 10, 2007 4:18 PM
	To: t10 at
	Subject: Persistent Reservation Proposal - Group Reservations
	I have posted two documents related to an additional Persistent
Reservation Type.  The first document is a presentation on where
persistent reservations are today and where they fall short in the
scenarios covered by the proposal.  It also covers the intent of the
proposal and what will be proposed.  The second is the actual proposal 
	Your PDF file will be posted at:
	Normally, the posting/archiving process takes about 30 minutes. 
	Kevin D. Butt
	SCSI & Fibre Channel Architect, Tape Firmware, IBM
	MS 6TYA, 9000 S. Rita Rd., Tucson, AZ 85744
	Tel: 520-799-2869 / 520-799-5280
	Fax: 520-799-2723 (T/L:321)
	Email address: kdbutt at 

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