July 15, 2015

Universally Unique Identifier (UUID)


A Universally Unique IDentifier (UUID) is a 16-octet (128-bit) number. In its canonical form, a UUID is represented by 32 lowercase hexadecimal digits, displayed in five groups individually separated by hyphens, in the form 8-4-4-4-12. In general, UUID is used to uniquely identify an object or entity on the Internet.

VMware storage architecture has multiple, unique identifiers:
  • NAA & EUI (most common):
    • Network Address Authority  & Extended Unique Identifier
    • Guaranteed to be unique to the LUN
    • The preferred method of identifying LUNs
    • Generated by the storage device
  • MPX  (local datastores):
    • For devices that do not provide an NAA number, ESXi generates an MPX identifier
    • Represents the local LUN or disk
    • Takes the form mpx.vmhba<Adapter>:C<Channel>:T<Target>:L<LUN>, e.g. mpx.vmhba33:C0:T1:L0
    • Can be used in the exact same way as the NAA identifier
  • VML: Can be used interchangeably with the NAA identifier and the MPX identifier
    • Generally used for operations with utilities such as vmkfstools
  • Path identifiers, e.g. vmhba1:C0:T0:L1
    • Used exclusively to identify a path to the LUN
    • Generally used for operations with utilities such as vmkfstools
  • Target identifiers, e.g. fc.200100e08ba5ff63:210100e08ba5ff63
We are now adding UUID – Universally Unique IDentifier to the list.

"In addition to being universally unique, no centralized authority is required to administer them."

In vSphere, the UUID is a 16-octet (128-bit) number. It is represented as 16-hexidecimal number pairs. The 16 bytes of this value are separated by spaces, except for a dash between the eighth and ninth hexadecimal pairs.

An example UUID looks like this:
uuid.bios = "00 11 22 33 44 55 66 77-88 99 aa bb cc dd ee ff"

VMware uses the UUID to generate a MAC address for a VM.

The UUID value is based on the physical host’s System Management BIOS (SMBIOS) and the path to the virtual machine’s configuration (.vmx) file.

The UUID is stored in the SMBIOS system information (the BIOS of the VM) descriptor and in the file system superblock.

Within the VMX configuration file the UUID information is stored in three variables: uuid.bios, uuid.location and vc.uuid.
  • uuid.bios
    • globally unique identifier
    • generated when a VM is powered on or reset
  • uuid.location
    • hash based on the current path of the VM
    • generated whenever the VM is migrated
  • vc.uuid
    • used by vCenter to identify VM
    • generated when you add VM to inventory (or create VM
The UUID value must be surrounded by quotation marks. A sample configuration is shown below:
Associating a UUID with a virtual machine allows that virtual machine to be uniquely identified even if its network configuration is changed.

Note: VMware uses the form 8-8-4-12 instead of the formal 8-4-4-4-12 form.


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