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194 Configuration: Device Mapper Multipath for Linux
• 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
CPU 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 CPU 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 been updated with proper
settings. After this, run multipath. Then run
multipath –ll, the new devices should 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 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 194 Thursday, December 9, 2010 3:20 PM