[Libguestfs] building a supermin appliance with febootstrap...

[Richard W.M. Jones]

On Wed, Aug 17, 2011 at 04:50:17PM +0530, Kashyap Chamarthy wrote:
> Ok, so , If I have to build appliance which has a bunch of %post,
> %preun, %postun, %pre operations in it's RPM SPEC file, I need to
> pull all of those into a custom init script(like the one for
> libguestfs[1]), so that the guest kernel can detect this(run it as
> PID 1) and sanely bring up the rest of the system ?

Yes, you're gonna need an /init file anyway.

Another unfortunate thing about Fedora is that the base RPMs like
"filesystem-*.rpm" don't contain all the base files.  If you look at
the source for mock, anaconda and so on, you'll see some code to
create the base system.  For example:

https://fedorahosted.org/mock/browser/py/mock/backend.py#L385

Naturally this duplicated code is slightly different in each program
where it appears.

So your /init file should probably create some basic filesystem and
device nodes as in the mock example above, then include any bits from
the %pre/%post scripts that look important, and then jump to whichever
init system you want (eg. /sbin/init which in newer Fedora is a link
to systemd).

> And then I need to invoke 'febootstrap-supermin-helper' like below to build the appliance.
> 
> # febootstrap-supermin-helper -f ext2 supermin.d x86_64 kernel initrd appliance.
> 
> Where appliance is the file system.  As of now, other file systems
> like ext3/ext4(maybe btrfs in future) are not yet available ?

It would be very simple to modify febootstrap to support ext3 and/or
ext4.  (btrfs would definitely NOT be a simple change).  In libguestfs
the appliance is added read-only with a disposable overlay on top.
Thus journals (a data integrity feature) don't make any sense for us.

http://git.annexia.org/?p=libguestfs.git;a=blob;f=src/launch.c;h=e58add54362b7933ab28e023bff492462d8ceee0;hb=HEAD#l632

> Question: After I build and boot the appliance, how about networking
> inside it? Any pointers to that?

You just need to pass the right parameters to qemu.  Assuming you are
using a real init system, the network will be brought up automatically
inside the guest when it detects the virtual hardware.

http://git.annexia.org/?p=libguestfs.git;a=blob;f=src/launch.c;h=e58add54362b7933ab28e023bff492462d8ceee0;hb=HEAD#l596

Note also that you can overlay any extra files (eg. config files,
programs) that you need in the appliance by dropping cpio files into
the supermin.d directory.  Again, take a look at how libguestfs does it:

http://git.annexia.org/?p=libguestfs.git;a=tree;f=appliance;hb=HEAD

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
virt-p2v converts physical machines to virtual machines.  Boot with a
live CD or over the network (PXE) and turn machines into Xen guests.
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