Does OS alert when SIZE shows full or SIZE ON DISK

We use a product called Commvault to archive files on our file servers and if you go by the reporting tool we use (Orion Solarwinds) it looks like nothing has been archived and reports that the disk is full because it is reporting on SIZE and not SIZE ON DISK. We could probably tweak the tool to query the size on disk rather than size which would be ok but my main concern is what the OS thinks it has available. If i have a 200GB drive and i archive 20GB of data, the size reads 200GB but the size on disk reads 180GB which one of those does the OS use? I am just worried that even though there is 20GB free, the OS will give that alert about how the drive is low on space and then stop users from saving files to that drive. Which would mean it "looks at" size and not size on disk.
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It should alert on SIZE ON DISK since that represents the sum of the clusters used on the disk. The SIZE figure is the amount of data inside those clusters. If a file is using even part of a cluster, the whole cluster is shown as being used. So for NTFS with a 4K cluster size, a 1K file will occupy the 4K cluster and "use" all 4K of space.
uwaadminAuthor Commented:
Yeah that is what i figured but i was hoping someone could confirm this for sure. Many companies are archiving their data these days so i assumed someone would be able to verify this from their experience.
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