Tag Archives: febootstrap

febootstrap is now called supermin

The new supermin home page is here, tarballs are available here, and the new git repository is here.


Filed under Uncategorized

What’s the maximum number of virtio-blk disks?

It depends on the number of other PCI devices that you are exporting to the guest. In the current implementation each virtio-blk disk is a separate PCI device (although Anthony pointed out that you can create multifunction PCI devices which let you break this limit in some narrow circumstances).

How many other PCI devices are you likely to have? Run lspci in a guest to see. Probably you have a host bridge, an ISA bridge for all your legacy devices, a network card, a VGA adapter, a USB controller, an emulated PIIX4 and a memory balloon. That’s 7 devices, and we haven’t even got to disks yet.

PCI is limited to 32 devices in total. So 25 or so virtio block devices is the maximum you would be able to add to a real guest without accepting limitations on things like hotplugging which you would get if you used multifunction devices. (libguestfs is limited to 25 disks but for other reasons to do with limits in febootstrap).

Work is proceeding to remove this limit.

1 Comment

Filed under Uncategorized

Tip: Creating throwaway appliances with febootstrap

This is an update to a previous posting, but using the new febootstrap 3.x cross-distro toolchain to make building supermin appliances even simpler.

Firstly we create a supermin appliance containing a few packages and their dependencies. Note that I’m not minimizing this appliance so it’s a bit bigger (3.1MB!) than the ones we would generate in reality:

$ mkdir supermin.d
$ febootstrap --names 'bash' 'coreutils' -o supermin.d
febootstrap: warning: some host files are unreadable by non-root
febootstrap: warning: get your distro to fix these files:
$ ls -lh supermin.d/
total 3.1M
-rw-rw-r--. 1 rjones rjones 2.7M Dec 10 18:23 base.img
-rw-rw-r--. 1 rjones rjones 462K Dec 10 18:23 hostfiles

The purpose of these two files is explained in the febootstrap documentation.

This won’t boot without a /init script, and we can easily provide one:

$ cat init
#!/bin/bash -
echo Welcome to my world
bash -i
$ chmod +x init
$ echo init | cpio -o -H newc --quiet > supermin.d/init.img

The 3 files in supermin.d/ are a supermin appliance, and could be packaged up in a Fedora, Debian or Ubuntu package.

When you actually want to come and launch this appliance, you use febootstrap-supermin-helper to reconstruct the appliance:

$ febootstrap-supermin-helper -f ext2 supermin.d x86_64 \
    kernel initrd root

(Note that “kernel”, “initrd” and “root” are output files in that command)

You’d usually arrange for those files to be cached, since febootstrap-supermin-helper takes a few seconds to run (8 seconds on my laptop) and by caching it you can get reconstruction time down to a fraction of a second.

Now to boot, run qemu or qemu-kvm like this:

$ qemu-kvm -kernel kernel -initrd initrd -hda root

After a few seconds you’ll get to the shell:


Filed under Uncategorized

libguestfs Debian packages

Up to date Debian packages (*.deb) for libguestfs are now available:


Please read the README file first.


$ time guestfish -a /dev/null run : supported
      augeas yes
     inotify yes
 linuxfsuuid yes
linuxmodules yes
 linuxxattrs yes
        luks yes
        lvm2 yes
       mknod yes
      ntfs3g yes
   ntfsprogs yes
    realpath yes
       scrub yes
     selinux yes
          xz yes
    zerofree yes

real	0m17.335s
user	0m16.061s
sys	0m0.608s

Note that time is running Debian inside a virtual machine. On native hardware with KVM that command should run much more quickly.

Leave a comment

Filed under Uncategorized

An explanation of supermin appliances

I wrote an explanation of supermin appliances.


Filed under Uncategorized

Tip: Pack files into a new disk image

Note: This requires libguestfs ≥ 1.4

If files/ is a directory containing some files, you can create a new disk image populated with those files using the command below (virt-make-fs can also be used for this):

$ guestfish -N fs:ext2:400M -m /dev/sda1 tar-in <(tar -C files/ -cf - .) /


  1. guestfish -N fs:ext2:400M creates a prepared disk image which is 400MB in size with a single partition formatted as ext2. The new disk image is called test1.img.
  2. -m /dev/sda1 mounts the new filesystem when guestfish starts up.
  3. tar-in [...] / uploads and unpacks the tar file into the root directory of the mounted disk image.
  4. <(tar -C files/ -cf - .) is a bash process substitution which runs the tar command and supplies the output (ie. the tar file) as the input on the command line.

This is a quick way to create “appliances” using febootstrap and libguestfs, although you should note that I don’t think these appliances would really work, I just use them for testing our virtualization management tools, like the ability to detect and manage disk images:

$ febootstrap -i fedora-release -i bash -i setup -i coreutils fedora-13 f13
$ echo '/dev/sda1 / ext2 defaults 1 1' > fstab
$ febootstrap-install f13 fstab /etc 0644 root.root
$ febootstrap-minimize f13
$ guestfish -N fs:ext2:40M -m /dev/sda1 tar-in <(tar -C f13 -cf - .) /
$ guestfish --ro -i -a test1.img

Welcome to guestfish, the libguestfs filesystem interactive shell for
editing virtual machine filesystems.

Type: 'help' for a list of commands
      'man' to read the manual
      'quit' to quit the shell

Operating system: Fedora release 13 (Goddard)
/dev/vda1 mounted on /
><fs> cat /etc/fstab
/dev/sda1 / ext2 defaults 1 1


Filed under Uncategorized

LXC howto

KageSenshi’s HOWTO use Linux Containers (LXC) on Fedora 12 with libvirt is interesting. I discovered that they’re using febootstrap (see earlier postings) to build the Fedora root filesystem for the containers.


Filed under Uncategorized