Dell PowerVault MD3820f - Manual (Page 201)

Troubleshooting
Question Answer
How can I check if multipathd is
running? Run the following command.
/etc/init.d/multipathd status
Why does the multipath –ll
command output not show any
devices? First verify if the devices are discovered or not. The command
#cat /proc/scsi/scsi displays all the devices that are
already discovered. Then verify the multipath.conf to ensure
that it is been updated with proper settings. After this, run
multipath. Then run multipath –ll, the new devices must
show up.
Why is a newly-mapped LUN not
assigned a multipathing device node? Run rescan_dm_devs in any directory. This should bring up the
devices.
I removed a LUN. But the
multipathing mapping is still
available. The multipathing device is still available after you remove the
LUNs. Run multipath –f <device node for the deleted
LUN> to remove the multipathing mapping. For example, if a
device related with /dev/dm-1 is deleted, you must run
multipath –f /dev/dm-1 to remove /dev/dm-1 from DM
mapping table. If multipathing daemon is stopped/restarted, run
multipath –F to flush out all stale mappings.
Failback does not happen as
expected with the array. Sometimes the low level driver cannot auto-detect devices
coming back with the array. Run rescan_dm_devs to rescan
host server SCSI bus and re-aggregate devices at multipathing
layer.
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