Disks and Reservations

Gerry Houlder gerry.houlder at seagate.com
Tue Jun 25 14:23:10 PDT 2013

Formatted message: <a href="http://www.t10.org/cgi-bin/ac.pl?t=r&f=r1306252_f.htm">HTML-formatted message</a>

Yes, reservations are intended to limit access to a particular logical unit
to either one initiator or a group of initiators. The idea is to prevent
access to initiators that are not part of the reservation. A reservation
can be "write only" (i.e., anyone can read but only the reservation holder
can write) or exclusive (only the reservation holder can read or write). An
initiator that is permitted through a reservation may access any location.
The random access nature of disks means that there isn't any concern about
preserving a particular "location of the R?W head".
Preempting is intended to allow another initiator to take over for a failed
initiator that holds a reservation (thus locking others out) and needs to
be replaced by a new initiator. Aborting permits the new initiator to abort
all of the commands associated to the failed initiator, so they don't
consume resources.
I don't think there are any Unit Attentions related to reservations that
are ignored. If an initiator is denied access due to a reservation, that
initiator's commands are terminated with RESERVATION CONFLICT status. This
doesn't fall into the Unit Attention category (i.e., a CHECK CONDITION
status with a Unit Attention sense key).
On Tue, Jun 25, 2013 at 3:30 PM, Kevin D Butt <kdbutt at us.ibm.com> wrote:
> I am struggling to understand Reservations in the disk world.  Me being a
> tape person, my mind set is wrapped around protecting the logical position
> of the tape. This is does not seem to be a concern for disks.
> What are the reasons reservations are used in the disk world?  I assume it
> is to protect a Logical Unit for one applications sole use (at least
> writing).  Are there other conceptual reasons?
> What are the reasons that a PREEMPT would be used?  In the tape world,
> PREEMPT is typically only used to perform a fail-over from a lost I_T nexus
> to an alternate path.  Is this the same in the disk world, or are there
> other reasons?
> Also, sometimes in the tape world, Unit Attentions are ignored.  Is this
> the same in the disk world, specifically related to reservations?
> Thanks,
> Kevin D. Butt
> SCSI & Fibre Channel Architect, Tape Firmware
> Data Protection & Retention
> MS 6TYA, 9000 S. Rita Rd., Tucson, AZ 85744
> Tel: 520-799-5280
> Fax: 520-799-2723 (T/L:321)
> Email address: kdbutt at us.ibm.com
> http://www-03.ibm.com/servers/storage/

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