I think we are talking about different things
- a VMFS is the file system which is used to hold all files of a VM (as you are a beginner - please do not take offense - I ignore more complex configurations). It should use the full space of the virtual disk.
You don't need 300 GB? Well, then create a smaller virtual disk! Or keep the free space for additional VMs or snapshots. Never ever plan to fill a VMFS to 100%!!
- lets say you create a Windows VM and want put the OS on the C: drive with 20GB.
In that case you create a 20GB 'container file' with a '.vmdk' file type and map it to the first SCSI address (0:0) in the VM.
- You need 180 GB for the data? Then create a second '.vmdk' and map it to (0:1). Windows believes it has 2 (two) physical SCSI disk drives.
You can quite easily expand the second '.vmdk', as long as you don't store a pagefile on it. And with some work you can even expand the C: disk without backup/restore or partition games.
- The remaining 100 GB of the VMFS from the EVA virtual disk are unused - well, not completely:
-- there is VMFS meta-data which takes a few-100 MB
-- some small files that describe the VM and hold logs
-- the virtual machine swap file which by default is as large as the VM's memory.
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