vSphere 5.5 - Datastore shows 20GB free. Disk in Windows has 800GB free?

I have a datastore that's showing it has 20GB free space out of 1,000 GB provisioned space.
However, only thing on this datastore is a virtual disk connected to a VM.
The disk in a Windows server VM is just a disk that's the 1T disk, and it has 800GB free.

Why would the Windows free space show 800GB, but the datastore in vSphere is 20GB free?
Anyway I can free up the space to reflect?
I'm finding the datastore has insufficient space for a backup.

It is Thick Provision Lazy Zeroed.
garryshapeAsked:
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Norm DickinsonConnect With a Mentor GuruCommented:
Hmmm...seems to me that If you have provisioned  a 1 TB disk space for a VM, that space is not going to be available as free space. You need to provision a more realistic amount to free up some space, like say 400 GB or so, vs. 1 TB. Then the difference will become available.
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garryshapeAuthor Commented:
The difference would become available in vSphere? or in Windows?

Windows shows 800GB free for the disk.
vSphere shows 20GB free for the datastore the disk is on.
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Norm DickinsonConnect With a Mentor GuruCommented:
The difference would be available on the host machine.
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Andrew Hancock (VMware vExpert / EE MVE^2)Connect With a Mentor VMware and Virtualization ConsultantCommented:
When you allocate a virtual machine disk (vmdk) it's allocated and uses all the space immediately on the datastore.

So when you look at the Host OS, you have 20GB free. (which is extremely LOW!), and should be 20-30% left free on the datastore.

Your Windows VM OS, has 800GB free to use.
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garryshapeAuthor Commented:
Got it, I am sorry. I was forgetting that the disk was the same size of the datastore.

I will just not make it as large as the damn datastore anyways. not sure why so much space was requested in the first place if they're not even using it! story of my life.    

As far as "Veeam" goes which is my backup software. Do you know if I actually exclude this disk from the backup of this VM, will Veeam still make a snapshot and still run the risk of the excluded disk saving files to itself that may fill it up?
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Andrew Hancock (VMware vExpert / EE MVE^2)Connect With a Mentor VMware and Virtualization ConsultantCommented:
All backup software uses the Storage API, and snapshot mechanism to backup VMs.

if you exclude a disk, it should not create a snapshot for it.

and snapshots only fill up datastores, because the backup software has failed to work correctly, but it does happen, and your responsibility as VMware Admin to monitor and check! after backups VMs do not remain on snapshots!
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