[CentOS] scsi3 persistent reservations in cluster storage fencing

I'm interested in the idea of sharing a bunch of SAS JBOD devices 
between two CentOS servers in an active-standby HA cluster sort of 
arrangement, and found something about using scsi3 persistent 
reservations as a fencing method.    I'm not finding a lot of specifics 
about how this works, or how you configure two initiator systems on a 
SAS chain.   I don't have any suitable hardware for running any tests or 
evaluations yet.

general idea:  2 centos servers each with 8 port external SAS cards (2 
x4), cascaded through a SAS box-o-disks with a whole bunch of SAS dual 
ported drives, to implement high availability large nearline 
storage.       all storage configured as JBOD, using linux md raid or 
lvm mirroring. drives should only be accessible by the currently active 
server, with heartbeat managing the fencing.

here's a hardware block diagram
http://freescruz.com/shared-sas.jpg


This is about the only details I've found on scsi persistant 
reservations and linux HA
http://docs.redhat.com/docs/en-US/Red_Hat_Enterprise_Linux/5/html/Configuration_Example_-_Fence_Devices/SCSI_Configuration.html

One question I have is: how well will this scale with several strings of 
100 SAS drives on the same HA pair of servers?

Can SAS storage instead be fenced at the SES/expander level rather than 
having to use reservations with each separate drive?

-- 
john r pierce                            N 37, W 123
santa cruz ca                         mid-left coast