Installation guide

UUID and Other Persistent Identifiers
125
If there are multiple paths from a system to a device, device-mapper-multipath uses the WWID to
detect this. Device-mapper-multipath then presents a single "pseudo-device" in /dev/mapper/
wwid, such as /dev/mapper/3600508b400105df70000e00000ac0000.
The command multipath -l shows the mapping to the non-persistent identifiers:
Host:Channel:Target:LUN, /dev/sd name, and the major:minor number.
3600508b400105df70000e00000ac0000 dm-2 vendor,product
[size=20G][features=1 queue_if_no_path][hwhandler=0][rw]
\_ round-robin 0 [prio=0][active]
\_ 5:0:1:1 sdc 8:32 [active][undef]
\_ 6:0:1:1 sdg 8:96 [active][undef]
\_ round-robin 0 [prio=0][enabled]
\_ 5:0:0:1 sdb 8:16 [active][undef]
\_ 6:0:0:1 sdf 8:80 [active][undef]
Device-mapper-multipath automatically maintains the proper mapping of each WWID-based device
name to its corresponding /dev/sd name on the system. These names are persistent across path
changes, and they are consistent when accessing the device from different systems.
When the user_friendly_names feature (of device-mapper-multipath) is used, the WWID is
mapped to a name of the form /dev/mapper/mpathn. By default, this mapping is maintained in the
file /var/lib/multipath/bindings. These mpathn names are persistent as long as that file is
maintained.
Warning
The multipath bindings file (by default, /var/lib/multipath/bindings) must be available at
boot time. If /var is a separate file system from /, then you must change the default location of
the file. For more information, refer to http://kbase.redhat.com/faq/docs/DOC-17650.
Important
If you use user_friendly_names, then additional steps are required to obtain consistent
names in a cluster. Refer to the Consistent Multipath Device Names
1
section in the Using Device-
Mapper Multipath
2
book.
In addition to these persistent names provided by the system, you can also use udev rules to
implement persistent names of your own, mapped to the WWID of the storage. For more information
about this, refer to http://kbase.redhat.com/faq/docs/DOC-7319.
21.3.2. UUID and Other Persistent Identifiers
If a storage device contains a file system, then that file system may provide one or both of the
following:
• Universally Unique Identifier (UUID)
1
http://www.redhat.com/docs/en-US/Red_Hat_Enterprise_Linux/5.4/html/DM_Multipath/multipath_consistent_names.html
2
http://www.redhat.com/docs/en-US/Red_Hat_Enterprise_Linux/5.4/html/DM_Multipath/index.html