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

[Kevin Wolf]

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