Clarification for Persistent Reservation + All Registrants

Nicholas A. Bellinger nab at
Fri Dec 12 17:53:15 PST 2014

* From the T10 Reflector (t10 at, posted by:
* "Nicholas A. Bellinger" <nab at>
On Sat, 2014-12-13 at 00:52 +0100, James Bottomley wrote:
> On Fri, 2014-12-12 at 14:59 -0800, Nicholas A. Bellinger wrote:
> > Hello T10/SCSI folks,
> > 
> > A question came up on the LIO target-devel mailing list recently wrt the
> > proper handling of Persistent Reservations + All Registrants logic in a
> > couple of different scenarios.
> I can tell you how it was originally designed over a decade ago to
> support clustering products (being in the business then of negotiating
> with the manufacturers).
> > The crux of the confusion stems from how registrations are treated
> > before an explicit PR-OUT RESERVE service action with All Registrants
> > type occurs, and after that explicit PR-OUT RESERVE of All Registrants
> > type has been released.
> > 
> > My original interpretation was that an explicit PR-OUT RESERVE with All
> > Registrants type had to first occur before other non reservation holding
> > registrations (existing or not) would gain the implicit reservation.
> You seem to be asking in the presence of registrations would an
> unregistered initiator receive a reservation conflict before the PR_OUT
> RESERVE is issued?  The answer, categorially is no because there's no
> reservation until a registered initiator establishes one.
Not exactly.  The question is if a RESERVE with type=All Registrants
still needs to happen in order for the other registrations to have their
'implicit' reservation activated.
As mentioned below, the reasoning was because I'd only ever seen RESERVE
populate the reservation TYPE field (eg: not REGISTER), which would
imply that a 'explicit' RESERVE needs to happen from one initiator in
order for the target to know if All Registrants logic needs to kick in
for other existing + future registrations.
> > This was primarily because the reservation type was not known until the
> > explicit PR-OUT RESERVE occurred, and AFAICT at the time there where no
> > host implementations that actually specified the reservation type at
> > registration time.
> > 
> > So Is it correct to assume that one explicit PR-OUT RESERVE with All
> > Registrants type needs to occur before other non reservation holding
> > registrations gain an implicit reservation..?
> I don't understand what you mean implicit reservation.  Once an all
> registrants reservation is established, *all* registered initiators
> become the reservation holder ... they're all equal.	This is mandated
> in SPC-3 5.6.9
Implicit reservation meaning a registration for an I_T nexus to a device
with a different I_T nexus holding an All Registrants reservation.
Eg, an I_T Nexus issued a RESERVE with type=All Registrants, and now the
other I_T nexus with an active registration on the same device have an
implicit reservation.
> > How about when the explicit reservation is removed..?  Do the implicit
> > reservations need to remain in effect for the existing registrations,
> > even though the explicit reservation of All Registrants type no longer
> > exists..?
> Again, I don't understand the distinction between implicit and explicit.
> For an all registrants reservation, all registered initiators are the
> reservation holder so any one of them may release the reservation.  Once
> the reservation is released, there's no reservation at all.
So some initiator still needs to perform the actual RESERVE, right..?
That is, a number of registrations don't become 'implicit' reservation
holders until one initiator performs an 'explicit' RESERVE with type=All
> > The second point that Ilias raised is what should be the correct
> > R_HOLDER bit status for PRIN READ_FULL_STATUS descriptors when All
> > Registrants type is specified.  My original interpretation was that
> > R_HOLDER was to signal the explicit reservation holder obtained with
> > PR-OUT RESERVE, and not for the implicit reservations from the other
> > registrations.  Is this the correct approach..?
> R_HOLDER is set for any initiator that would be defined to be the
> persistent reservation holder in section 5.6.9
Well, section 5.6.9 only mentions the use of RESERVE, so that would seem
to indicate the original interpretation + current code for
READ_FULL_STATUS is correct.
> > Also, does anyone have any experience with a host side implementation of
> > All Registrants to use as a reference for what's already out in the
> > wild..?
> I know what was negotiated to support clustering (which is a pretty tiny
> subset).  However, I think your point of confusion is trying to
> distinguish between what you call implicit and explicit reservations ...
> there's no such thing.  There's only a reservation holder which may be
> many initiators, So it's binary: a given initiator either is or is not
> the reservation holder.  Section 5.6 needs to be read in that context.
The confusion is around when registrations who don't issue an explicit
RESERVE become implicit reservation holders.  Is it when one initiator
does an explicit RESERVE + type=All Registrants, or is it when REGISTER
+ type=All Registrants occurs..?
* For T10 Reflector information, send a message with
* 'info t10' (no quotes) in the message body to majordomo at

More information about the T10 mailing list