[Libguestfs] Re: libguestfs performance

[Richard W.M. Jones]

On Tue, Dec 01, 2009 at 04:08:38PM +0100, Pierre Riteau wrote:
> For a project of mine I would like to create VMs from tarballs of
> root filesystems. I would like to use libguestfs since this is
> exactly what the tar2vm.sh guestfish recipe does.
>
> However, I need to extract tarballs that are in the hundreds of
> megabytes, but my tests showed that performance is very low, both
> with tgz-in and with guestmount (sorry I don't have the numbers any
> more, but it was something like more than one hour to extract a
> tarball containing a Debian Lenny installation).
>
> Is there any way to drastically improve libguestfs' performance?

First of all, read:

http://libguestfs.org/FAQ.html#slow

Using tar-in/tgz-in to copy the tarball into the appliance is always
going to be slow.  I would think you should get better performance if
you write the tarball into an ISO first, and attach that.  Something
like:

mkisofs -JR -o cd.iso the_tarball.tar.gz

guestfish -a guest.img -a cd.iso
[...]
><fs> mkmountpoint /guest
><fs> mkmountpoint /cd
[...]
><fs> mount /dev/sdb /cd
[...]
><fs> debug sh "tar xcf /cd/the_tarball.tar.gz /guest"

However we found experimentally that the fastest method of all for
creating distributions was to prebuild an ext2/3/4 image, and 'dd' it
over to the target partition, then resize it to the final (larger)
size.  This method is extremely quick, enabling us to build Fedora
images in just a minute or two:

http://rwmj.wordpress.com/2009/11/23/prebuilt-distributions-part-2/#content

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
libguestfs lets you edit virtual machines.  Supports shell scripting,
bindings from many languages.  http://et.redhat.com/~rjones/libguestfs/
See what it can do: http://et.redhat.com/~rjones/libguestfs/recipes.html