Re: [Qemu-devel] [PATCH] Add tar container format

[Alexander Graf]

On 14.08.2009, at 00:57, Anthony Liguori wrote:

> Alexander Graf wrote:
> > For that we still need to enable the qemu blockery to support  
> > stacking though.
> >
> > Signed-off-by: Alexander Graf <address@hidden>
>
> This feels like a bit too much of a one-off to me.  I'm concerned  
> that it's something we'd have to support long term that wouldn't  
> have very many users beyond your particular use-case.

Well, the same goes for "bochs", "parallels", "dmg" and all the other  
fun image format backends nobody uses, right? How do one or two more  
hurt you there?

Though I'm not saying that I wouldn't like to see users of this. It's  
great to have this option for archiving virtual machines (incl. config  
file) without the need to worry about readability of it. Just tar xzf  
it and you're done.

Also, we will start using this in SUSE Studio. So all appliances  
you'll download from there will be in tar.dzip format. That means that  
if this was in upstream, even fedora or ubuntu users could start those  
appliances without extracting or even downloading them first.

> So far, we only support image formats that are dedicated to virtual  
> machines.  We support http and nbd but those are generic protocols,  
> not necessarily special formats.  tar and dzip seem arbitrary.  Why  
> not zip, rar, bzip2, or any other format?

Simply because this was the only real standard I've found. The dict  
project uses dictzip for a couple of years now and it seems to be  
pretty stable (only a single version exists).

Bzip2 is supposed to be chunkable, but I haven't found anyone who did  
this yet and my knowledge in compression is pretty small. I don't know  
what rar does, but pkzip has a 2 GB file size limit (which is pretty  
bad for VM images) and I'm not aware of any seek extensions to it.

If you have good suggestions, please go ahead. We need a VM format  
container that

  a) everyone can extract with existing tools
  b) can be used as input for qemu

As a sidenote, the OVF specification states that OVF Packages are  
supposed to be TAR files.

> You could do all of this by making use of a fuse filesystem.

Right, but I really don't want to have a server serving a virtual  
machine rely on anything even close to fuse. What if the fuse client  
starts hanging? The whole box goes down? No thanks.

> I'd almost rather see something like gio integration so that this  
> sort of generic filesystem stuff could live somewhere else.  I'm  
> curious what others think though.  Does it seem reasonable to  
> include this type of functionality?


A plugin infrastructure sounds nice at first, but doesn't do all the  
fedoras and ubuntus out there too much good ;-).

Alex