Available Bug

Started 1 week ago | Questions thread
Tom_N Forum Pro • Posts: 15,584
Re: Available Bug
2

This sounds more like an optimization than a bug.

High Sierra supports APFS, and I'd guess that your internal SSD uses it.  APFS supports clones.  They "allow the operating system to make efficient file copies on the same volume without occupying additional storage space."  (Wikipedia)  It's a new optimization not found in HFS+.

When you duplicate a 2 GB file on an internal SSD, APFS doesn't allocate new disk space or copy all 2 GB worth of data.  It just creates a new filename and notes that the original and copied files share the same data.  That saves time and space – as long as all you are doing with the files is storing and reading them.

When you change a file that is using shared data, the system determines that sharing all of the data is no longer appropriate.  It allocates disk space so it can start storing data separately.  Then your space goes down by (in your case) 2 GB.

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