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

[Dietmar Maurer]

> > 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. 

really?

> 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.

Zero region are distributed, not continuous, in most cases.

> 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.

We track zero regions at 4K level, and Cluster size is 64K.

So I normally just use 1bit to store information about empty blocks.

I thought that is quite good, but you obviously have a better idea?