Re: [Libguestfs] [libguestfs] RFE: virt-builder option to auto-detect image format (#19)
On Mon, Dec 28, 2015 at 03:36:53PM -0800, Ryan Sawhill Aroha wrote:
Hi there
The repo definition at http://libguestfsorg/download/builder/index
contains `format=raw` data for each template, eg:
```
[centos-6]
name=CentOS 66
osinfo=centos66
arch=x86_64
file=centos-6.xz
revision=6
checksum=fc403ea3555a5608a25ad30ce2514b67288311a7197ddf9fb664475820f26db2bd95a86be9cd6e3f772187b384a02e0965430456dd518d343a80457057bc5441
format=raw
size=6442450944
compressed_size=199265736
expand=/dev/sda3
notes=CentOS 66
```
I haven't looked at the source, but it seems that `virt-builder`
doesn't do anything with that metadata
According to `man virt-builder`, it always assumes `--format raw`
The format parameter in the metadata and the --format option are
related, but different things.
The format= metadata tells virt-builder the format of the original
template (ie. centos-6.xz is raw format after decompression). Image
format auto-detection is a security risk [although to be fair really
not in this particular case], and so it's better to specify the
format.
The --format parameter is what format the virt-builder end user would
like your final image to be.
So if format=raw and --format qcow2, then virt-builder knows it must
convert the image. This is the code:
https://github.com/libguestfs/libguestfs/blob/master/builder/builder.ml#L...
The --format command line parameter defaults to raw (and this is
documented in the manual). It doesn't matter what the original
template format was.
Why? I've built my own repo with all xz-compressed qcow images
and
it seems crazy to me that `virt-builder` can't auto-detect this;
instead, I have to specify `--format qcow2` (or else vb converts
them from qcow2 to raw)
So virt-builder is behaving as per specification above.
My initial thought was that we could have an `output_format=...' line
in the metadata. However that has a couple of problems:
(1) Other metadata format like SimpleStreams doesn't support this.
(2) It breaks the virt-builder contract because we have documented in
the manual that the default output format (ie. if --format is not
specified) is raw.
However, read on ...
Halp! Can we either change the default behavior to look for
`format=qcow2|raw` in the template definition if `--format` isn't
specified, or if you're unhappy changing the behavior, could we add
`--format=auto`?
We couldn't call it "auto", since --format=auto is used in other
virt-* tools contexts to mean to do unsafe autodetection of the
format.
I don't mind if we called it something else, perhaps `--format=input'?
It just has to be a name that could never be used as a real disk
format (like 'qcow2' or 'vmdk').
The RFE process is described here:
http://libguestfs.org/guestfs-faq.1.html#how-do-i-propose-a-feature
https://bugzilla.redhat.com/enter_bug.cgi?component=libguestfs&produc...
Rich.
--
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
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