Re: [Qemu-devel] [PATCH v4 4/6] introduce new vma archive format

[Dietmar Maurer]

> > 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.