Administrator Guide

Table 15. Useful device mapper commands
Command Description
multipath –h
Prints usage information.
multipath –ll
Displays the current multipath topology using all available information—sysfs, the device
mapper, path checkers, and so on.
multipath
Reaggregates multipathing device with simplified output.
multipath –f
<multipath_dev_node>
Flushes out Device Mapper for the specified multipathing device. Used if the underlying
physical devices are deleted/unmapped.
multipath –F
Flushes out all unused multipathing device maps.
rescan_dm_devs
Dell EMC provided script. Forces a rescan of the host SCSI bus and aggregates multipathing
devices as needed. Use this command when:
• LUNs are dynamically mapped to the hosts.
• New targets are added to the host.
• Failback of the storage array is required.
• For MD Series Dense iSCSI storage arrays, iSCSI sessions have to be established for
rescan to take effect.
Limitations and known issues
• In certain error conditions with the no_path_retry or the queue_if_no_path feature is set, applications may hang. To
overcome these conditions, enter the following command for each affected multipath device:
dmsetup message [device] 0 "fail_if_no_path"
where [device] is the multipath device name—for example, mpath2; do not specify the path
• I/O may hang when a Device Mapper device is deleted before the virtual disk is unmounted.
• If the scsi_dh_rdac module is not included in initrd, slower device discovery may be seen and the syslog may become populated
with buffer I/O error messages.
• I/O may hang if the host server or storage array is rebooted while I/O is active. All I/O to the storage array should be stopped before
shutting down or rebooting the host server or storage array.
• With an MD Series storage array, after a failed path is restored, failback does not occur automatically because the driver cannot
autodetect devices without a forced rescan. Run the command rescan_dm_devs to force a rescan of the host server. This restores
the failed paths enabling failback to occur.
• Failback can be slow when the host system is experiencing heavy I/O. The problem is exacerbated if the host server is also
experiencing high processor utilization.
• The Device Mapper Multipath service can be slow when the host system is experiencing heavy I/O. The problem is exacerbated if the
host server is also experiencing high processor utilization.
• If the root disk is not blacklisted in the multipath.conf file, a multipathing node may be created for the root disk. The command
multipath –ll lists vendor/product ID, which can help identify this issue.
• If upgrading from a previous version of SLES, uninstall and then reinstall the latest scsi_dh_rdac module on the updated SLES
installation. Then update the kernel and install the MD Storage Manager from the DVD.
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