Hi,
How can I see what kind of preallocation uses a qcow2 file? I tried with this command: but the type of pre-allocation (off; full; metadata; falloc; etc) is not shown. Thank you.

Preallocation affects only how the QCOW2 file is created, not how it is later used, and that detail is not recorded in the QCOW2 header. I don't know of any easy way to discover it later. For the data clusters, you can look at the "qemu-img map --output=json" output and see if there are any clusters that have a non-zero offset field but show "false" in the data field. If there are any such clusters, then the preallocation was not "falloc" or "full".

For the metadata area, the only way I can think of is to use "hdparm --fibmap" and look for holes in the metadata area. The presence of holes wpuld indicate that the metadata was not preallocated.

I don't know of any way to distinguish between "falloc" and "full", nor can I think of any reason you would need to know that.

I don't get the question. You asked "what kind"?

The info option only has this.

info [-f fmt] filename
Give information about the disk image filename. Use it in particular to know the size reserved on disk which can be different from the displayed size. If VM snapshots are stored in the disk image, they are displayed too.