linux-ext4 - Re: [PATCH 2/2] e2image: add -a switch to include all data

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Date:	Fri, 17 Feb 2012 10:17:43 +0100 (CET)
From:	Lukas Czerner <lczerner@...hat.com>
To:	Phillip Susi <psusi@...ntu.com>
cc:	"Ted Ts'o" <tytso@....edu>, linux-ext4@...r.kernel.org
Subject: Re: [PATCH 2/2] e2image: add -a switch to include all data

On Thu, 16 Feb 2012, Phillip Susi wrote:

> -----BEGIN PGP SIGNED MESSAGE-----
> Hash: SHA1
> 
> On 02/16/2012 06:17 PM, Ted Ts'o wrote:
> > As I recall Lukas disclaimed a guarantee that the code would work on
> > qcow2 images that weren't generated by e2image.  (In particular, it
> > definitely doesn't support compressed or encrypted qcow2 images.)
> > 
> > So we need to make sure we add the appropriate disclaimers that this
> > might not work on qcow2 images generated by tools other than e2image.
> 
> Would you care to craft such a disclaimer?
> 
> > It will only work for raw images.  The QCOW2 format uses an entirely
> > different code path, since we don't have an QCOW2 io_manager
> > abstraction.  That was my original hope, but that's not how our qcow2
> > support was implemented, so it won't work, and we should probably give
> > a reasonable warning if someone tries to use the -a flag with anything
> > other than a raw file system image for e2image's input.
> 
> It seemed to work fine with qcow2 when I tested it.

So the problem actually is that I have made some assumptions, like for
example that all data are laid out sequentially (because it is how we
are creating the qcow2 images with e2image) and possibly more. It also
does not support encrypted and compressed images, but it will warn you
about that. And most importantly I have never tested other qcow2 images
other than those generated with e2image, simply because that this is not
the intention of this tool (you have qemu to do that). But if you're able
to test it reliably and create a test case so we can be sure that it
actually works, then we would not need such disclaimer I guess.

Note that you'll have to test that qcow2 images generated by e2image
actually works in other tools reliably (qemu) (again this was not the
intention, we just used qemu2 format because it is space efficient and
non-sparse, so te image can be easily transferred), and that e2image
works well with images generated by other tools. I am not saying that it
would not work (I guess it should), I just never tested it.

The good thing about the qcow2 -> raw conversion done by e2image is that
it is *a lot* faster than qemu implementation (at least when I tested
it). I have never tried to find out why, unfortunately I have not even
tried to see what could be fixed in qemu (I really should:)).

Thanks!
-Lukas
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