Dell PowerVault MD3200i - Manual (Page 200)

198 Configuration: Device Mapper Multipath for Linux
• After a failed path is restored on an MD3600i Series array, failback does not
occur automatically because the driver cannot auto-detect 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 very 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 very 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.
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 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
must bring up the devices.
I have no LUNs mapped before.
Then I map some LUNs. After
running rescan-scsi-bus.sh, LUN 0
doesn’t show up.
Run rescan_dm_devs instead of rescan-
scsi-bus
for LUN 0 reconfiguration.
book.book Page 198 Tuesday, June 18, 2013 3:15 PM