Information Technology Reference
In-Depth Information
This VMFS datastore is hosted on an active-active storage array; the currently assigned
policy is Fixed (VMware), which is the default for an active-active array.
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This VMFS datastore resides on the i rst LUN hosted by an EMC VNX array. This is noted
by the LUN column and also the L1 in the runtime name.
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To change the multipathing policy, simply select a new policy from the Path Selection Policy
drop-down list and click OK. One word of caution: Choosing the wrong path selection policy
for your specii c storage array can cause problems, so be sure to choose a path selection policy
recommended by your storage vendor. In this particular case, the Round Robin policy is also
supported by active-active arrays such as the EMC VNX hosting this LUN, so we'll change the
path selection to Round Robin (VMware).
Changes to the path selection are immediate and do not require a reboot.
We're nearing the end of the discussion of VMFS datastores, but we do need to cover two
more topics. First, we'll discuss managing copies of VMFS datastores, and then we'll wrap up
this discussion with a quick review of removing VMFS datastores.
Managing VMFS Datastore Copies
Every VMFS datastore has a universally unique identii er (UUID) embedded in the i le system.
When you clone or replicate a VMFS datastore, the copy of the datastore is a byte-for-byte copy,
right down to the UUID. If you attempt to mount the LUN that has the copy of the VMFS data-
store, vSphere will see this as a duplicate copy and will require that you do one of two things:
Unmount the original and mount the copy with the same UUID.
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Keep the original mounted and write a new signature to the copy.
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Other storage operations might also cause this behavior. If you change the LUN ID after
creating a VMFS datastore, vSphere will recognize that the UUID is now associated with a new
device (vSphere uses the NAA ID to track the devices) and will follow this behavior.
In either case, vSphere provides a GUI in the Add Storage Wizard that allows you to clearly
choose which option you'd like to use in these situations:
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Choose Keep Existing Signature if you want to mount the datastore copy without writing
a new signature. vSphere won't allow UUID collisions, so you can mount without resig-
naturing only if the original datastore has been unmounted or no longer exists (this is the
case if you change the LUN ID, for example). If you mount a datastore copy without resig-
naturing and then later want to mount the original, you'll need to unmount the copy i rst.
Choose Assign A New Signature if you want to write a new signature onto the VMFS
datastore. This will allow you to have both the copy and the original mount as separate
and distinct datastores. Keep in mind that this process is irreversible—you can't undo the
resignaturing operation. If the resignatured datastore contains any VMs, you will likely
need to reregister those VMs in vCenter Server because the paths to the VM's coni guration
i les will have changed. The section “Adding or Registering Existing VMs” in Chapter 9
describes how to reregister a VM.
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Let's take a look at removing a VMFS datastore.
