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Inside a VSS (Shadow Copy on a Microsoft OS) a file takes space according to data changes since the VSS was done (this is simplified, for example not true if a later VSS was done).
If I understand well, the space increment is 1 cluster, so if 1 byte was changed then 1 cluster is allocated for that file into the VSS.

How can I know what space a random file consume into a VSS?
For example a database file fills 1 GiB on the live filesystem, but can only fills 100 KB into a VSS because few data changed. Is there a way to check that?

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  • What do you mean by "cluster"? There is no mention of anything like that in the documentation.
    – harrymc
    Dec 12, 2020 at 18:36

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Shadow Copy on Windows works per-volume, because it operates at the block level of the disk/volume. It saves blocks that are changed in the volume, so doesn't distinguish between folders, the Master File Table (where folders and files are defined), or just plain file data.

As there is no connection between VSS and the files or folders on the disk, just blocks, there is no way to find out how many shadow blocks relate to one specific file. Especially as the blocks currently on the disk may not correspond to the same file on the VSS backup or snapshot.

The answer is then negative - this is not possible. It would require keeping the history of the blocks between the time that the snapshot was taken and the current state of the disk. But a mechanism for doing that does not exist.

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