VMware View 4.5: Rebalance

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Rebalance: A desktop rebalance operation that evenly redistributes linked-clone desktops among available datastores.

When testing this, customers can often see unexpected results. This is often due to their misconception of how this function of View works. 

Rebalancing

Why? View 4.5 looks for the datastore/s that have the highest "weighted available space". The formula that View 4.5 uses to calculate this is:

weighted_available_space = datastore_capacity * overcommit_factor – virtual_usage

datastore_capacity = As you would expect, the size of the datastore.
overcommit_factor = The Storage Overcommit setting applied to each datastore when selecting which datastores to use for your desktop pools.

  • None – Storage is not overcommitted.
  • Conservative – x4 the size of the datastore. (This is the default level.)
  • Moderate – x7 the size of the datastore
  • Aggressive – x15 the size of the datastore.

virtual_usage = The maximum possible usage of all existing VMs on this datastore, which is calculated by adding up current sizes of all disks and configured sizes of memories of all existing VMs on the datastore.

Examples

Rebalance Examples

  • DS1 – 1000GB (Datastore Size) * 4 (Conservative Overcommitment) – 0 (No VM's deployed) = 4000
  • DS2 – 1000GB (Datastore Size) * 4 (Conservative Overcommitment) – ((20GB + 130MB)x5) (5 VM's already deployed) = 3865
  • DS3 – 1000GB (Datastore Size) * 7 (Moderate Overcommitment) – ((20GB + 130MB)x5) (5 VM's already deployed) = 6865

 

Common Scenario

You have a desktop pool with DS1 as it's only datastore. You then decided to add a second datastore, DS2. You perform a Rebalance on the pool to spread the VM's across the two datastores, but nothing happens… Why? Because DS2's weighted_available_space is less than that of DS1's.

  • If you increased the Overcommit level on DS2 to Moderate (x7) and ran Rebalance, the VM's would all migrate completely over to DS2 because it's weighted_available_space would be more than that of DS1 even though there are already 5 VM's on DS2. (See DS3 example)
  • If you Storage vMotioned the 5 VM's off of DS2 and ran Rebalance, the VM's in the pool should evenly distribute themselves across both DS1 and DS2.

Summary

As you can see, by default you may not get your desired effect. But with a little bit of maths you can get the reaction you want from the Rebalance feature. I must stress, this is only for VMware View 4.5 and older. 

 
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