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VMDK size on VSAN datastore

Hi All,

Just getting started with VSAN and have a question. I noticed on a VM that has a 100GB vDisk that the VMDK actually shows as 200GB when I look at the VSAN datastore. I have FTT=1, so I understand there will be 2 copies of the VM (200GB) I guess I don't completely understand why they exist in a single VMDK. I would have expected to see 2 - 100GB vDisks, not 1 -200GB vDisk. Can someone shed some light on this? Is it because we can't have two VMDKs with the same name?


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VMware Employee
VMware Employee

Like you mentioned, the reason you see it as 2x its size, its because of the FTT=1. vSAN is an object storage, and those objects are comprised of components based on the policy you use. For a default policy of FTT=1, you will have one object for your 1 VMDK, in this case your 100GB VMDK, and it will have 3 components. 2 of those components are copies of your data (100GB each) per FTT=1, and the other component is a small witness component for that particular object.

Each VM has a potential for 5 types of objects:

  1. Namespace
  2. VMDK
  3. Swap object
  4. Snapshot
  5. Snapshot Memory

Hope this helps

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Hello Pratt,

When you create an FTT=1 Object (a vmdk here) this object is typically made of 3 components in a R0 configuration - copy of disk + copy of disk + a small metadata Witness component (used as a tie-breaker).

If the disk is larger than 255GB (and/or you specify Styripe Width > 1), it will ~double the number of components (basically a R1 on top of a R0)

You can view the state, layout and physical location of these components using RVC vsan.vm_object_info <path to VM>

or using cmmds-tool via the CLI

or from the Web client by selecting the VM > VM Storage Policies > Select a Disk and the component layout appears in the bottom pane.


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