> > We currently only need 8 bytes per cluster, which results in small files. > > So how big is the metadata overhead? If you always copy 64k at once (which is > a > very conservative assumption - at least for the background copy you'll have > much larger blocks), then an 8 byte header for each is 0.01%. Increasing that > to > 0.02% doesn't sound like a huge problem to me.
We do not always write 64K, because we omit regions filled with zeroes. So the picture gets a bit worse when you have sparse images. For example, if you backup a file which mostly contains empty blocks, you will duplicate the size of the resulting file! And this is a very common case. Opposed to that, VM images >= 1Petabyte are very uncommon.