Persistent Reservation Proposal - Group Reservations

Kevin D Butt kdbutt at
Thu Jan 3 10:51:06 PST 2008

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We looked at using the existing reservation key but could not make it work 
because, among other things, it is returned in PR In and it's behavior is 
assumed to be non unique by existing applications.  For these reasons we 
did not spend too much time looking at it.
IBM has one intent, but I found that there are at least two others, Roger 
Cummings and Fred Knight, that have different intents.	Roger was thinking 
of bringing something similar in himself - though it is subtly different. 
I am not sure I understand all those subtleties.
The basic intent I was aiming for is for use in the tape drives (making 
sure it is also good for disks).  There is a need to have any reservation 
solidly exclusive to a given host.  However, it is desirable to allow 
multiple HBA's on the same host to have that exclusive access.	This 
allows for load balancing and failover internal to the same host.  Add to 
that the need to allow for failover to a second host - also with multiple 
HBA's but guaranteeing that if whatever out of band communication is 
occurring between he two hosts fails that the two hosts will be protected 
|from each other.  That is, if somehow each host thinks it should have the 
access, the tape drive is still protected such that only one host at a 
time can have permission to access the drive.  If host A thinks host B 
went away and steals the reservation (a la PREEMPT) host B will no longer 
have permission and will get a UA next time it tries access.  Add to this 
the ability for the tape drive to be shared in an open system and still 
have this work.
In reality, this is nothing more than each HBA on a single host being able 
to share an exclusive reservation and no other initiator port can join in 
without the owner telling it how to.
I sent another note planning a phone conference so we can clearly lay out 
the desired intent.  Please respond to that note.
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 
Black_David at 
Sent by: owner-t10 at
01/03/2008 07:19 AM
<Gerry.Houlder at>, <t10 at>
RE: Persistent Reservation Proposal - Group Reservations
* From the T10 Reflector (t10 at, posted by:
* Black_David at
I'm still checking with our implementers, but my initial reaction
is that Gerry's 2-for-2:
(1) I strongly agree with Gerry's preference for a new RESERVATION
		 type over a new REGISTRATION type.  PR code is already
		 subtle in a number of ways, and a new reservation type is
		 much easier to isolate from existing types in 
		 coding, and testing.  It should also have better behavior
		 when mixing implementations that don't all support the
		 new functionality.  We probably need two new types, one
		 for group write and one for group exclusive.
(2) Is there a reason why we can't use the existing reservation
		 key as the entity that has to match for the new 
		 type?	One would have to ensure that the reservation key
		 can't be read from the device by PR IN, but we already
		 have the precedent (and the specification text) to do
		 that in the All Registrants reservation types.  Reuse of
		 the reservation key is going to be easier to cope with
		 than inventing a new group identifier.
What initiator implementations are likely to use this new
functionality for what purposes?
David L. Black, Distinguished Engineer
EMC Corporation, 176 South St., Hopkinton, MA  01748
+1 (508) 293-7953	      FAX: +1 (508) 293-7786
black_david at	   Mobile: +1 (978) 394-7754
> -----Original Message-----
> From: owner-t10 at [mailto:owner-t10 at] On Behalf 
> Of Gerry.Houlder at
> Sent: Wednesday, January 02, 2008 4:40 PM
> To: t10 at
> Subject: RE: Persistent Reservation Proposal - Group Reservations
> * From the T10 Reflector (t10 at, posted by:
> * Gerry.Houlder at
> *
> So this is Roger's preference:
> >1) Define a new type of REGISTER that contains one (or more) GRIDs;
> >2) Extend RESERVE to include a single GRID, and only Initiators that
> registered (or will register) with a matching GRID get access under
> reservation;
> >3) Don't change PREEMPT, superseding reservations etc. at all - keep
> rules for handling the reservation keys completely orthogonal to the
> >4) Don't allow GRIDs to be accessed via PR In.
> If we don't add a new reservation type, it sounds like you are
describing a
> new registration type (existing one without GRID, new one with GRID).
> seems to complicate the rules for making an "all registrants"
> If the initiator making the reservation specifies a GRID, only other
> initiators that registered with that GRID value inherit the
reservation and
> the others are excluded. This would be a very different result for
> initiators that expect to inherit any reservations just by
> Further, the "unexpectedly excluded" registered initiator will get no
> as to why it doesn't inherit the reservation. if it issues READ
> it gets back a response that says an all registrants reservation is in
> force. The PR generation and reservation key values returned won't
help any
> either, and the GROUP ID value of course will not be returned.
Creating a
> new reservation type would help here, since the reservation type is
> returned by READ RESERVATION. This might be an advantage of creating a
> reservation type for GRID Registrants Only (or something like that).
> new reservation type would make it more obvious why you didn't inherit
> reservation you expected to inherit.
> Also for backwards compatibility, will initiators be able to register
> existing method (i.e., not specifying a GROUP ID) as well as a new
> that specifies a GROUP ID? The standard will have to describe how
> registrants without a group ID, registrants with a non-matching group
> and registrants with a matching group ID will interact with the
> registrants only reservation and a "GRID registrants only"
> Do we really lose that much if we simply create a "matching
> key" reservation instead? That means only initiators that register
with a
> matching reservation key inherit the reservation. We lose a little bit
> security because the matching ID value is publicly reported and a
> initiator can still join, but a malicous initiator could probably get
> around the Group ID value match just by bute force retries anyway (how
> bytes of group ID is enough?), or just preempt the group ID
reservation and
> replace it with a regular registrants only reservation. This is a
> method of creating a Group ID reservation that probably works just
fine for
> co-operating initiators (as opposed to previously described malicious
> ones).
> *
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