> Yes, try it with dd and cp (GNU version only?):
> dd if=/dev/zero of=/tmp/zero-test count=1000
> cp --sparse=always /tmp/zero-test /tmp/zero-sparse
> ls -l /tmp/zero-test /tmp/zero-sparse
> du -cs /tmp/zero-test /tmp/zero-sparse
> What I do not know is how many fs support it, and if they can do on
> the fly or a forced copy is needed

It is the copy which makes the sparse file.  You can't make a hole in a file
merely by writing a bunch of zeros to it.  You can only do it by seeking
past the (current) end of the file, then writing non-zero data.  The bytes
you seeked over are the hole, and will be read as if zeros.

GNU cp uses a bunch of heuristics to discover runs of zeros in the input
file and seek over them in the output file, rather than just writing zeros.


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