On Mon, Mar 11, 2013 at 09:26:26AM +0000, Dietmar Maurer wrote: > > > > We track zero blocks at 4KB level to get small backup files. Restore > > > > can handle those blocks differently, either pre-allocate or create > > > > holes. That > > > can even be a restore option. > > > > > > You are right, the behavior can be set at restore time. > > > > > > I'm curious how much of a win the 4 KB zero detection is on random > > > Linux or Windows guest images. Have you collected numbers? > > > > Well, seems that it does not help much - it saves about 0.2% space. So maybe > > we can remove that feature (and increase address space in the VMA format > > instead). > > Sorry, I need to correct myself - I had a bug in the code. > > I can see space reduction up to 4% using this feature. Considering the fact > that it > comes at no cost, it would be stupid to remove it.
Okay, looks like it's useful but not a huge win. In the NBD approach pipelining writes (or discards) ought to make 4 KB zero blocks usable without overhead. Stefan