[Nottingham] Copy without copying?

[Andy Smith]

Hello,

On Fri, Jun 07, 2019 at 03:12:21PM +0100, J J via Nottingham wrote:
> 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

You already found overlayfs. Another possible answer is btrfs or
xfs, which support copy on write.

By using either of those you can do cp --reflink=always … and it
will do a copy that only adds a reference to the source, not an
actual copy of the data. This uses no additional space. If you alter
one of the copies then the link is broken.

And of course btrfs also has writeable snapshots.

Cheers,
Andy

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