Why multipath cannot recognize any of the EMC SAN Luns?

Issue

• In order to tune 'rr_min_io' value for EMC SYMMETRIX array on the RHEL 5.6 host, following entries were made in /etc/multipath.conf file:

devices {
  device {
       vendor EMC
       product SYMMETRIX
       path_grouping_policy multibus
       getuid_callout /sbin/scsi_id -g -u -ppre-spc3-83 -s /block/%n
       rr_min_io 100
  }
}

The changes for rr_min_io parameter were immediately reflected and were verified using 'multipathd -k' interactive console.

# multipathd -k
> > show config
> > CTRL-D

However when the system is rebooted, multipath cannot recognize any of the EMC SAN Luns. Removing the customized entry and restarting multipathd service brings back the EMC SAN Lun.