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Using the vssadmin list shadowstorage command in Windows 7, one gets an overview of the disk space related to shadow copies. Typically three values are returned:

  • Used Shadow Copy Storage space
  • Allocated Shadow Copy Storage space
  • Maximum Shadow Copy Storage space

However, I have a hard time to understand the logic/difference between the three.

In particular:

  • I see in System Protection that I can tune the Max Usage, but then where does the allocated space comes from and how would it be different of the used space?
  • And if the user has the hand on the allocated space, how can one assign this value and why then wouldn't the system use the maximum space available?
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The meaning of these values is as follows :

  • Maximum : The amount of space that can be allocated
  • Allocated : The amount that is currently allocated that can grow up to the Maximum
  • Used : The amount that is used within the Allocated area.

Numerically, we have the following inequality :

Used ≤ Allocated ≤ Maximum

All values can grow up to the Maximum. Manual intervention is required to increase the Maximum by using the vssadmin resize shadowstorage command.

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  • Yes, this is how I understand the terms. But I still don't understand the difference between "used" and "allocated" because those terms seem synonymous to me. Could it be that "used" is the amount of data stored and "allocated" is the amount of space those data take up on the filesystem in terms of blocks (same as when one looks at the properties of a file)? Also is the "maximum" space set completely reserved and inaccessible to the user even if not completely used or allocated?
    – The Quark
    Commented Nov 16, 2022 at 22:34
  • I agree with your formulations. About whether the Maximum space is reserved, this does not seem to be the case. Microsoft's documentation does not answer this question, but user posts say so : link1 and link2.
    – harrymc
    Commented Nov 17, 2022 at 8:14
  • ... It stands to logic, that if the Maximum was reserved, there would be no reason for not allocating immediately all of it.
    – harrymc
    Commented Nov 17, 2022 at 8:15
  • So, according to this website: learn.microsoft.com/en-us/previous-versions/technet-magazine/…, 'allocated' means 'reserved'. That was the missing piece for my understanding. Mabe you wish to edit your answer accordingly and I make it as accepted?
    – The Quark
    Commented Nov 17, 2022 at 11:04
  • Your article says exactly what is written in my answer, that 'allocated' does not mean 'reserved'. Quote: "Lists the volume shadow storage space currently in use, the space that is reserved for future use (labeled as allocated), and the maximum space that might be dedicated."
    – harrymc
    Commented Nov 17, 2022 at 14:56

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