Am 22.02.2013 um 14:11 hat Dietmar Maurer geschrieben: > > > 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!
If describing an empty image takes more than a couple of bytes, you're doing something seriously wrong. Because for that case it would be easy enough to have an entry that just says "0 GB - 2 GB" is zero, and that's it. 16 bytes for 2 GB of virtual disk size, sounds pretty good to me. And it would only be needed if VMAs support backing files, because for normal sparse blocks, I don't even see any reason why the VMA should contain any information about them. They are never written to, so they are by definition sparse. Kevin
