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

[Kevin Wolf]

Am 22.02.2013 um 12:21 hat Dietmar Maurer geschrieben:
> > > For now I have no plans to backup such large VMs.
> > 
> > 640k ought to be enough for anybody?
> > 
> > Code is easy enough to change that "works for now" can be good enough.
> > Changing file formats and external interfaces is hard, though, so better 
> > get it
> > right the first time. I don't want to get patches for a new format in a 
> > year or two
> > just because 128T was enough for the first few users.
> 
> The limits are not really arbitrary - I choose them carefully to keep the 
> overhead small.
> 
> 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.

And I think some other reasons were already suggested why using 16 bytes
would be better, so maybe we should just do it.

Kevin