Huge difference for the free space shown on Hyper-V host and on VM itself.

On the Hyper-V host (Pic 1 and Pic 2), we have a volume called “Volume2” containing 4 x 4TB HDs (mirrored) -- so the total capacity is about 7.27 TB. This volume2 provides for nothing but the VM file server “BUD” – with the following two VHDX files:

Bud.vhdx (2012R2 OS) :     about 52.35GB of max 80GB (dynamic)
Bud_Data.vhdx (for Data): about  4.49TB  of max 7.27TB (dynamic)

So we can do a math and figure the free space of Volume2 is roughly about 2.67TB as shown on Pic 1.
But if we go into the VM’s drive properties and Disk Management, we can see the free space of that drive is about 4.58TB.  That is a huge difference. Can you please educate me why and what could be wrong?
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When you have defined as starting point 4.49TB for the VM, you can see the remaining 2.67TB free in the host. Inside the VM you have 4.58TB free space which is free for the VM, but the host is reading the VHDX file, not the free space into the file.

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David Johnson, CD, MVPOwnerCommented:
note you have physical 7.27TB
you have oversubscribed using dynamic expanding VHDX's  (80GB + 7.27TB) = 8.07TB
Bud.vhdx (2012R2 OS) :     about 52.35GB of max 80GB (dynamic).
Bud_Data.vhdx (for Data): about  4.49TB  of max 7.27TB (dynamic) -- 2.78 remaining virtual
Physical 5.0135 USED - 7.27 = 2.26 TB remaining

remember that once a drive expands it doesn't contract unless you shrink it so you have probably deleted some data from Bud_Data
you have allocated 4.49 TB for data drive, however since this is dynamically expanding disk, it consumes disk space which is actually needed
according to figures provided by you, if we calculate, 7.27 (actual physical disk size) - 4.58 (remaining free disk space on physical disk) =2.69 TB space is consumed  on physical host by both vhdx files irrespective of what size you specified for vhdx file in VM properties
Host cannot see what you have allocated at VM level, it only concerned about actual physical space consumed on physical disk
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CastlewoodAuthor Commented:
Thank you guys' input.
For the over-subscription issue David brought out, I got an question... Isn't it right the data loss by over-subscription issue shall NOT occur until the free space is running out? Look the volume2 providing for BUD.vhdx and BUD_Data.vhdx has about 2.67TB free space while BUD.vhdx is actually just with 80GB in size, which is over-subscribed. The large free size should be able to accommodate the very small over-subscription? Yes or No??
In another word, should I avoid over-subscription whatsoever in a dynamic expanding environment ?
David Johnson, CD, MVPOwnerCommented:
You have to be more aware of the remaining space when you oversubscribe. There is a penalty when the disk grows as the file is expanded by the virtual machine management. If you have several dynamically expanding volumes then you can run into disk fragmentation problems.  If the virtual machine manager can't allocate space then that virtual machine is stopped as saved  (critical).. Try and keep the free space in the 25%-30% range.
David Johnson, CD, MVPOwnerCommented:
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