174 Configuration: Device Mapper Multipath for LinuxLimitations and Known Issues• In certain error conditions with the no_path_retry or thequeue_if_no_path feature is set, applications may hang. To overcomethese conditions the you must enter the following command for eachaffected multipath device:dmsetup message [device] 0 "fail_if_no_path"where [device] is the multipath device name (for example. mpath2; donot specify the path)• I/O may hang when a Device Mapper device is deleted before the volumeis unmounted.• If the scsi_dh_rdac module is not included in initrd, slower devicediscovery may be seen and the syslog may become populated with bufferI/O error messages.• I/O may hang if the host server or storage array is rebooted while I/O isactive. All I/O to the storage array should be stopped before shutting downor rebooting the host server or storage array.• With the MD3200 Series array, after a failed path is restored, failback doesnot automatically occur because the driver cannot auto-detect deviceswithout a forced rescan. Run the command rescan_dm_devs to force arescan of the host server. This restores the failed paths enabling failback tooccur.• Failback can be slow when the host system is experiencing heavy I/O. Theproblem is exacerbated if the host server is also experiencing very highCPU utilization.• The Device Mapper Multipath service can be slow when the host system isexperiencing heavy I/O. The problem is exacerbated if the host server alsoexperiences very high CPU utilization.• If the root disk is not blacklisted in the multipath.conf file, a multipathingnode may be created for the root disk. The command multipath –lllists vendor/product ID which can help identify this issue.
