So, same as title, what is WWID ?
It stands for The World Wide Identifier (WWID ) which 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 WWID s to the current /d ev/sd names can be seen in the symlinks maintained in the /dev/disk/by-id/ directory.

WWID

For example, a device with a page 0x83 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 /dev/sd name on that system. Applications can use the /dev/disk/by-id/ 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, device-mapper-multipathuses 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-map per-multipath ) is used, the WWID is mapped to a name of the form /d ev/mapper/mpathn. By default, this mapping is maintained in the file /etc/multipath/bi ndings. These mpathn names are persistent as long as that file is maintained.
Important
If you use user_friendly_names, then additional steps are required to obtain consistent names in a cluster. Refer to the Consistent Multipath D evice Names in a Cluster section in the Using DM Multipath Configuration and Administration book.
In addition to these persistent names provided by the system, you can also use ud ev rules to implement persistent names of your own, mapped to the WWID of the storage.