[Nottingham] Copy without copying?

[J J]

I have some tests I'd like to run and in order to get them going the first
thing I can to do is copy a large(ish) folder structure that can be
read/written by the tests. This copy is then discarded.
The copying is wasteful in terms of time and disc space, so I'd like to
avoid having to do that.
Clearly any changes could be record as a difference, so my question is:

With Ext4, is there some magic way I can create and ad hoc "differencing
volume" that appear to be like a copy but isn't. The tests can do what they
like and that the whole thing gets chucked.
i.e. avoiding the copy operation

I'm pretty sure if we had ZFS or something this would be possible, but
that's not the case here. Wondering if FUSE or something can be used to
fake it.

My current reading suggests the answer is "No", but I thought I'd ask some
experts.

J.
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