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Re: [Qemu-devel] stacking the block layer

From: Kevin Wolf
Subject: Re: [Qemu-devel] stacking the block layer
Date: Wed, 12 Aug 2009 14:43:44 +0200
User-agent: Thunderbird (X11/20090320)

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.


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