An SBC-2 question regarding persistent reservation.

Tom Coughlan coughlan at missioncriticallinux.com
Fri Aug 18 14:01:59 PDT 2000


* From the T10 Reflector (t10 at t10.org), posted by:
* Tom Coughlan <coughlan at missioncriticallinux.com>
*
I heard about a question that came up during a recent SBC-2 review
regarding persistent reservations.  I though I would express my opinion,
and also use this opportunity to point out my change of address.
(Changed jobs, changed company, but can't get away from persistent
reservations. Lord have mercy.)

As I understand it, the question concerns why the text approved for
inclusion in SPC-2, SBC-n and SMC-n describes two different
requirements, one for commands that write the media, and one for
commands that do not:

 > A command that does not explicitly write the medium shall be checked
 > for reservation conflicts before the command enters the current task
 > state for the first time. Once the command has entered the current task
 > state, it shall not be terminated with a Reservation Conflict due to a
 > subsequent reservation. A command that explicitly writes the medium
 > shall be checked for reservation conflicts before the device server
 > modifies the medium or cache as a result of the command. Once the
 > command has modified the medium, it shall not be terminated with a
 > Reservation Conflict due to a subsequent reservation.

Why not just require that all commands shall be checked for reservation
conflicts before the command enters the current task state for the first
time?

I did not not write this section, but I believe that the purpose was to
give the implementor as much freedom as possible. An implementation that
checks every command "up front" will meet this specification, but there
are other legitimate implementations that should not be ruled out by the
Standard.  Over-specification can be very costly.

I would like to see the text stay the way it is, but if a change is
made, be sure to change it in SPC-2 (Rev. 18, Section 5.5.1) as well as
SBC-n and SMC-n.

--
Tom Coughlan
Mission Critical Linux
tel: 978-606-0262
coughlan at missioncriticallinux.com 

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