VNX: Error "multipath.conf line 16, invalid keyword: prio_callout" when setting up DM-MPIO on RHEL 6

When setting up Device Mapper-Multipath I/O (DM-MPIO) on Red Hat Enterprise Linux (RHEL) 6, the following error message is returned:

[root@IL-RDC-RH-77 ~]# multipathd -v2 -d
Mar 13 17:42:41 | --------start up--------
Mar 13 17:42:41 | read /etc/multipath.conf
Mar 13 17:42:41 | multipath.conf line 16, invalid keyword: prio_callout
Mar 13 17:42:41 | process is already running

[root@IL-RDC-RH-77 ~]# cat /etc/redhat-release
Red Hat Enterprise Linux Server release 6.2 (Santiago)

In the RHEL 6 release, priority functions are no longer callout programs. Instead, they are dynamic shared objects like the path checker functions. The prio_callout parameter has been replaced by the prio parameter.

[root@IL-RDC-RH-77 ~]# cat /etc/multipath.conf
defaults {
        user_friendly_names yes
}

blacklist {
        devnode "^(ram|raw|loop|fd|md|dm-|sda|sr|scd|st)[0-9]*"
        devnode "^hd[a-z]"
        devnode "^cciss!c[0-9]d[0-9]*"
}

devices {
        device {
        vendor "DGC"
        product "*"
        prio_callout "/sbin/mpath_prio_alua /dev/%n" << This option is not correct for this version of RHEL.
        path_grouping_policy group_by_prio
        features "1 queue_if_no_path"
        failback immediate
        hardware_handler "1 alua"
}
}

To address this issue, edit the /etc/multipathd.conf file.

Replace prio_callout "/sbin/mpath_prio_alua /dev/%n" with prio "emc" or prio "alua"