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?
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.
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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