Dell EMC PowerVault MD 34XX/38XX Series Storage Arrays Administrator's Guide

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Important information about special partitions

When using Device Mapper with the MD Series storage arrays, all physical disks are assigned a disk device node. This includes a special device type used for in-band management of the storage arrays, known as the Access Disk or Universal Xport device.

CAUTION: Certain commands, such as lsscsi, display one or more instances of Universal Xport devices. These device nodes must never be accessed, mounted, or used in any way. Doing so can cause loss of communication to the storage array and possibly cause serious damage to the storage array, potential making data stored on the array inaccessible.

Only multipathing device nodes and partition nodes created using the directions provided above must be mounted or in any way accessed by the host system or its users.

Table 1. Useful device mapper commands The following table provides the description for the commands:
Command	Description
multipath –h	Prints usage information.
multipath –ll	Displays the current multipath topology using all available information—sysfs, the device mapper, path checkers, and so on.
multipath	Reaggregates multipathing device with simplified output.
multipath –f <multipath_dev_node>	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 EMC 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.