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

[Kevin Wolf]

Am 22.02.2013 um 14:52 hat Dietmar Maurer geschrieben:
> > > 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.

Yes, but an image "which mostly contains empty block" has zero regions
that are way larger than 64k. Maybe you can't describe full 2 GB with 16
bytes, but quite sure some megabytes.

> > 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?

Maybe I didn't understand right then. These zero regions are just for
describing empty sectors within non-empty clusters? But why are you
concerned about mostly empty images then?

Kevin