Is "prio_callout readsector0" a valid entry in multipath.conf on Red Hat Enterprise Linux 5?

Issue

• While seting up Red Hat Cluster Suite, following entries were added in /etc/multipath.conf file:

  defaults {
  
  udev_dir                /dev
  polling_interval        5
  selector                "round-robin 0"              
  path_grouping_policy    failover             
  getuid_callout          "/sbin/scsi_id -g -u -s /block/%n"             
  prio_callout           readsector0             
  rr_min_io               100             
  max_fds                 max              
  rr_weight               priorities
  failback                immediate        
  no_path_retry           3user_friendly_names     yes
  
  }
  

• Is the entry "prio_callout  readsector0" correct?