Dell PowerVault MD3820f - Manual (Page 200)

Command Description
multipath –f Flushes out Device Mapper for the specified multipathing device. Used
if the underlying physical devices are deleted/unmapped.
multipath –F Flushes out all unused multipathing device maps.
rescan_dm_devs Dell provided script. Forces a rescan of the host SCSI bus and
aggregates multipathing devices as needed. Use this command when:
• LUNs are dynamically mapped to the hosts.
• New targets are added to the host.
• Failback of the storage array is required.
• For MD Series Dense iSCSI storage 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 an MD Series storage 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.
• 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.
• If upgrading from a previous version of SLES, uninstall and then reinstall the latest scsi_dh_rdac
module on the updated SLES installation. Then update the kernel and install the MD Storage Manager
from the DVD.
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