System information

In addition, path-based names are system-specific. This can cause unintended data changes when
the device is accessed by multiple systems, such as in a cluster.
For these reasons, several persistent, system-independent, methods for identifying devices have
been developed. The following sections discuss these in detail.
25.3.1. WWID
The World Wide Identifier (WWID) can be used in reliably identifying devices. It is a persistent, system-
independent ID that the SCSI Standard requires from all SCSI devices. The WWID identifier is
guaranteed to be unique for every storage device, and independent of the path that is used to access
the device.
This identifier can be obtained by issuing a SCSI Inquiry to retrieve the Device Identification Vital
Product Data (page 0 x83) or Unit Serial Number (page 0 x80 ). The mappings from these WWIDs to the
current /d ev/sd names can be seen in the symlinks maintained in the /d ev/d i sk/by-i d /
directory.
Examp le 25.3. WWID
For example, a device with a page 0 x83 identifier would have:
scsi-3600508b400105e210000900000490000 -> ../../sda
Or, a device with a page 0 x80 identifier would have:
scsi-SSEAGATE_ST373453LW_3HW1RHM6 -> ../../sda
Red Hat Enterprise Linux automatically maintains the proper mapping from the WWID-based device
name to a current /d ev/sd name on that system. Applications can use the /d ev/d i sk/by-i d /
name to reference the data on the disk, even if the path to the device changes, and even when
accessing the device from different systems.
If there are multiple paths from a system to a device, d evice- map p er- mu lt ipat h uses the WWID to
detect this. Device- map p er- mu lt ipat h then presents a single "pseudo-device" in
/d ev/mapper/wwi d, such as /d ev/mapper/36 0 0 50 8b4 0 0 10 5d f70 0 00 e0 0 0 0 0 ac0 0 0 0 .
The command mul ti path -l shows the mapping to the non-persistent identifiers:
Host: Channel: Target: LUN, /d ev/sd name, and the majo r: mi no r 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- map p er- mu lt ip at h automatically maintains the proper mapping of each WWID -based
device name to its corresponding /d ev/sd name on the system. These names are persistent across
path changes, and they are consistent when accessing the device from different systems.
⁠Chapt er 2 5. O nline St orag e Manag ement
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