I don't quite get how preallocating a sparse file helps. I see two reasons to preallocate: to suck up that space in the filesystem so you know you can get it when you need it, and to lessen fragmentation on the file. Preallocating a sparse file helps neither of these conditions. I'm sure it's quicker than writing zeros across 800MB, but if you're going to do that, why not just use the incremental method they list, which would generate sparse files anyway?