Why are there I/O reads from passive paths when using device-mapper-multipath on Red Hat Enterprise Linux?

Environment

• Red Hat Enterprise Linux 5 (RHEL5)
• SAN providing multiple paths to a storage device.

• device-mapper-multipath managing paths.

  • Either default or specifically configured to use directio or readsector0 path checkers.
  • Does not apply to multipath devices using TUR path checker.

  devices {
          # NetApp
          device {
                  vendor                  "NETAPP"
                  product                 "LUN"
                  path_grouping_policy    group_by_prio
                  features                "1 queue_if_no_path"
                  prio_callout            "/sbin/mpath_prio_alua /dev/%n"
                  path_checker            directio
                  path_selector           "round-robin 0"
                  failback                immediate
                  hardware_handler        "1 alua"
                  rr_weight               uniform
                  rr_min_io              128
                  getuid_callout          "/sbin/scsi_id -g -u -s /block/%n"
          }
  }