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Configuration: Device Mapper Multipath for Linux 193
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 volume
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 the MD3600i Series array, after a failed path is restored, 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.
rescan_dm_devs Dell provided script. Forces a rescan of the host SCSI bus
and aggregates multipathing devices as needed. For use
when:
• LUNs are dynamically mapped to the hosts.
• New targets are added to the host.
• Failback of the storage array is required.
• For MD3600i Series arrays, iSCSI sessions have to be
established for rescan to take effect.
Table 14-1. Useful Device Mapper Commands
(continued)
Command Description
book.book Page 193 Thursday, December 9, 2010 3:20 PM