[Qemu-devel] stacking the block layer

[Qemu-devel] stacking the block layer

[Alexander Graf]

Hi list,

I just implemented dzip (seekable gzip) support and stumbled across a  
major bummer in the block layer:

Formats can't be stacked.

So why do we have this restriction? From what I could tell, the  
current code would just run into an endless loop, because the probe  
function of the format we're trying to open triggers when trying to  
open the underlying file.

Now that is a valid problem, but can't we just match that when opening  
a file from say "qcow2" that we simply don't open backing files as  
"qcow2" as well?

That way we could have qcow2 in dzip served via HTTP. At the end of  
the day I also want to bring tar into the game, so we'd have qcow2  
(format) in tar (format) in dzip (format) on HTTP (protocol).

I'd like to hear your thoughts on that - are there any reasons not to  
go down this road?

Alex

Re: [Qemu-devel] stacking the block layer

[Kevin Wolf]

Alexander Graf schrieb:
> Hi list,
> 
> I just implemented dzip (seekable gzip) support and stumbled across a  
> major bummer in the block layer:
> 
> Formats can't be stacked.
> 
> So why do we have this restriction? From what I could tell, the  
> current code would just run into an endless loop, because the probe  
> function of the format we're trying to open triggers when trying to  
> open the underlying file.
> 
> Now that is a valid problem, but can't we just match that when opening  
> a file from say "qcow2" that we simply don't open backing files as  
> "qcow2" as well?
> 
> That way we could have qcow2 in dzip served via HTTP. At the end of  
> the day I also want to bring tar into the game, so we'd have qcow2  
> (format) in tar (format) in dzip (format) on HTTP (protocol).

No, this should actually be qcow2 (format) on tar (protocol) on dzip
(protocol) on http (protocol). I'm not sure though if you can specify
things like tar:dzip:http://... with the current block code but I don't
think you would run into the problem you described above with protocols.

Kevin