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[Libguestfs] Plans for changing libguestfs appliance building

Well you can all read the thread here for the technical details:
but the bottom line for anyone who wasn't in that discussion or on IRC
last week is that we have to change the way that the appliance is
built in libguestfs so that we don't depend (as much) on the qemu
-initrd option.  The bad thing is this is loads of make-work at a
particularly inconvenient time.  The good thing is that it should make
libguestfs 'boot' in under 5 seconds, even if your hardware is old and
slow.  It will also reduce the memory and I/O requirements for using

Because this issue is adversely affecting people using kernel 2.6.35
and/or the latest qemu right now (and will affect many people when
Fedora 14 is released which has both), this is my top priority to fix
in the coming week.

This email documents how I will change appliance building.  The
patches will follow some time later in the week when I've written and
tested them.

The new boot method will be this sequence:

(a) (Same as now) A suitable kernel to use is located on the host, or
from $libdir/guestfs in the non-supermin case.

(b) A tiny initrd is built (on the fly for supermin).  This will
contain just these files:

  any CD-ROM, IDE, virtio kmods required to read the virtual CD device
  ext2.ko if required
  a tiny init script, written in C

(c) An ext2 image is created for the root filesystem.  I will say a
lot more about how this is created below.

(d) qemu is invoked with something along these lines:

  qemu -kernel kernel -initrd initrd -drive file=isofile,media=cdrom

    kernel = kernel found in step (a)
    initrd = tiny initrd created in step (b)
    isofile = ext2 filesystem image created in step (c)

(e) The boot process proceeds by starting the kernel, reading the
/init script from the tiny initrd which mounts the filesystem from the
CD-ROM device and pivot_root(2)s into it, running another /init script
from that filesystem.  At this point, boot continues as it does in the
current libguestfs.

Since this involves attaching an extra device to the appliance, we
also need to change the daemon to ignore this extra device, adding
somewhat to the complexity of several operations.

We believe this should improve the speed of boot greatly.  Obviously
there is the saving because we are no longer using the (now broken)
qemu -initrd support with a large initrd, but that's purely because of
a qemu regression.  We also save because we don't need to unpack the
initrd inside the appliance, and because the device is loaded on
demand.  Also there are some unrelated changes that I intend to make
which will improve boot speeds.  We have every reason to believe that
5 seconds will be achievable, even on relatively old hardware.

Step (c) above involves creating an ext2 filesystem for root.  I chose
ext2 because it is considerably less complex than ISO9660, and is the
native Linux filesystem so it supports all Linux features (long
filenames, extended attributes and so on).

Creating an ext2 filesystem is more complex than the initrd that we
currently create using some hand-coded cpio-like code.  Of course we
can't use libguestfs to help us!  We plan to write some C code to
assemble the ext2 filesystem from scratch (or starting from a mke2fs
blank filesystem).

To further improve boot times, we intend to cache this so it will only
be created the first time it is used, and only when it needs to be
updated.  Caching this means that most times that you use libguestfs
or other tools, no appliance creation will be required at all, and
only the bits of the appliance that you use will be loaded.

If you have any further questions about this, please follow up on this
mailing list.


Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
virt-top is 'top' for virtual machines.  Tiny program with many
powerful monitoring features, net stats, disk stats, logging, etc.

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