Question about the use of PERSISTENT RESERVATION IN/OUT

[Brian Auld <bauld@lefthandnetworks.com>]

Sorry for the initial staggered email.

I was able to acquire some level of understanding of the use of "persistent
reservation in/out" commands after having read some SCSI specs and browsing
this mailing list's archives. I have made some presumptions and wanted 
to run
them by this list to make sure I wasn't too far out in left field.


I'm designing a linux-based application that is going to use LUNs for 
storage
from a backend FC SAN. It's not very sexy, but there is a one to one
relationship for i) boxes running my appl and ii) LUNs. One of the 
requirements
that has been imposed on the appl is that once a given backend LUN has been
assigned to my application, any other physical entity (i.e. a different
initiator on a different box, which could be running my appl) is to be
prevented from accessing this LUN using some kind of protection mechanism.


Once a LUN is assigned to a specific box running my appl, this protection
should exist until I explicitly deassign this LUN in my user interface. In
other words, the protection should persist across host reboots and target
reset/reboots.


I have made the following assumptions and decisions about how this will be
handled:


1) As I understand it, there is no reserve/release (persistent or old)
mechanism in the scsi subsystem itself.


2) WRT (1), there are however 2 mechanisms for sending pass-through commands
through the scsi subsystem directly to a target device: namely, (i) using an
sg device or (ii) sd ioctls.


3) What I would like to do is setup my application so the following 
occurs when
a LUN is assigned/deassigned to my appl:


assigned:
-------------
In my appl, send an appropriate combination of PERSISTENT RESERVE 
OUT(service
action: reserve ...) commands to effectivly lock this device so that any
initiator not included in the list I provide will be prevented from 
accessing
my LUN. I would use the sg mechanism or sd ioctls to do this.


deassigned:
---------------
In my appl, send an appropriate combination of PERSISTENT RESERVE OUT 
(service
action: release) commands to free up this LUN. Again, I would use sd ioctls
or the sg mechanism to do this.


Is this basic approach flawed? It would seem to be doable based on what 
I have
read so far, but I don't know for sure.


Any feedback would be appreciated.


Thanks,
Brian