What does qemu-img convert -W documentation mean?

[Richard W.M. Jones]

Firstly there's a real interesting thread about nbdcopy vs qemu-img
convert performance:
https://listman.redhat.com/archives/libguestfs/2021-May/thread.html#00119

I have a question about qemu-img convert -W flag.  The documentation
says:

 -W  Allow out-of-order writes to the destination.  This  option  im‐
     proves performance, but is only recommended for preallocated de‐
     vices like host devices or other raw block devices.

I don't understand why out of order writes are bad for things like raw
files and qcow2 files, or for non-preallocated block devices.
Wouldn't they always be a win?

Rich.

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