Important Information About Special Partitions; Limitations And Known Issues - Dell PowerVault MD3260i Administrator's Manual

1. Unmount all Device Mapper multipath device nodes mounted on the server: # umount
<mounted_multipath_device_node>
2. Stop the Device Mapper multipath service: # /etc/init.d/multipathd stop
3. Flush the Device Mapper multipath maps list to remove any old or modified mappings: # multipath –F
NOTE: The boot operating system drive may have an entry with the Device Mapper multipathing table. This is
not affected by the multipath –F command.
4. Log out of all iSCSI sessions from the host server to the storage array: # iscsiadm –m node --logout

Important Information About Special Partitions

When using Device Mapper with the MD Series Dense 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 6. Useful Device Mapper Commands
Command
multipath –h
multipath –ll
multipath
multipath –f
<multipath_dev_node>
multipath –F
rescan_dm_devs

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)
Description
Prints usage information.
Displays the current multipath topology using all available information (sysfs, the
device mapper, path checkers, and so on).
Re-aggregates multipathing device with simplified output.
Flushes out Device Mapper for the specified multipathing device. Used if the
underlying physical devices are deleted/unmapped.
Flushes out all unused multipathing device maps.
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.
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