Dell MD3600I, MD3620I owner manual Limitations and Known Issues, 193

Table 14-1. Useful Device Mapper Commands (continued)

Command	Description
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.

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.

Configuration: Device Mapper Multipath for Linux

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