[libguestfs.git] / README

Libguestfs is a library for accessing and modifying guest disk images.
Amongst the things this is good for: making batch configuration
changes to guests, getting disk used/free statistics (see also:
virt-df), migrating between virtualization systems (see also:
virt-p2v), performing partial backups, performing partial guest
clones, cloning guests and changing registry/UUID/hostname info, and
much else besides.

Libguestfs uses Linux kernel and qemu code, and can access any type of
guest filesystem that Linux and qemu can, including but not limited
to: ext2/3/4, btrfs, FAT and NTFS, LVM, many different disk partition
schemes, qcow, qcow2, vmdk.

Libguestfs provides ways to enumerate guest storage (eg. partitions,
LVs, what filesystem is in each LV, etc.).  It can also run commands
in the context of the guest.  Also you can access filesystems over
FUSE.

Libguestfs is a library that can be linked with C and C++ management
programs (or management programs written in OCaml, Perl, Python, Ruby,
Java, Haskell or C#).  You can also use it from shell scripts or the
command line.

Libguestfs was written by Richard W.M. Jones (rjones@redhat.com) and
hacked on by lots of other people.  For discussion, development,
patches, etc. please use the mailing list:

  http://www.redhat.com/mailman/listinfo/libguestfs


Home page
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  http://libguestfs.org/


Requirements
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- recent QEMU >= 0.12 with virtio-serial support

- febootstrap >= 2.9

- fakeroot

- fakechroot >= 2.9

- XDR, rpcgen (on Linux these are provided by glibc)

- pcre (Perl Compatible Regular Expressions C library)

- libmagic (the library that corresponds to the 'file' command)

- libvirt

- libxml2

- squashfs-tools (mksquashfs only)

- genisoimage / mkisofs

- (Optional) hivex >= 1.2.1 to build Windows Registry support

- (Optional) FUSE to build the FUSE module

- (Optional) Augeas (http://augeas.net/)

- perldoc (pod2man, pod2text) to generate the manual pages and
  other documentation.

- (Optional) Readline to have nicer command-line editing in guestfish.

- (Optional) xmllint to validate virt-inspector RELAX NG schema

- (Optional) OCaml + OCaml library xml-light if you want to rebuild
  the generated files, and also to build the OCaml bindings
  (http://tech.motion-twin.com/xmllight.html)

- (Optional) local Fedora mirror

- (Optional) Perl if you want to build the perl bindings

- (Optional) Python if you want to build the python bindings

- (Optional) Ruby, rake if you want to build the ruby bindings

- (Optional) Java, JNI, jpackage-utils if you want to build the java
bindings

- (Optional) GHC if you want to build the Haskell bindings

- (Optional) Perl XML::XPath, Sys::Virt modules (for libvirt support
in virt-inspector).

- (Optional, but highly recommended) perl-libintl for translating perl code.

- (Optional) po4a for translating manpages and POD files.

Running ./configure will check you have all the requirements installed
on your machine.


Building
----------------------------------------------------------------------

Then make the daemon, library and root filesystem:

  ./configure [--with-mirror=URI]
  make

Use the optional --with-mirror parameter to specify the URI of a local
Fedora mirror.  See the discussion of the MIRROR parameter in the
febootstrap(8) manpage.

Finally run the tests:

  make check

If everything works, you can install the library and tools by running
this command as root:

  make install


Fedora
----------------------------------------------------------------------

We provide packages for Fedora >= 11 in Fedora.  Use those, or build
from our source RPMs - it's far simpler that way.

You can compile libguestfs on Fedora 10 but you cannot use it with the
version of qemu in Fedora 10.  You need to compile your own qemu, see
section 'qemu' below.


RHEL / EPEL / CentOS etc
----------------------------------------------------------------------

We provide packages in EPEL which cover RHEL/CentOS >= 5.  Use those
or build from our source RPMs.


Debian
----------------------------------------------------------------------

libguestfs is now built as a package in Debian by Guido Gunther and
the other Debian libvirt maintainers.  See:

http://wiki.debian.org/Teams/DebianLibvirtTeam#Packages

You can build for Debian in two different ways, either building a
Fedora-based appliance using febootstrap, yum, rpm, fakeroot,
fakechroot (all packaged in Debian).  However the recommended way is
to build a Debian-based appliance using debootstrap and debirf.

Both ways are supported by the configure script.


qemu
----------------------------------------------------------------------

By far the most common problem is with broken or incompatible
qemu releases.

Different versions of qemu have problems booting the appliance for
different reasons.  This varies between versions of qemu, and Linux
distributions which add their own patches.

If you find a problem, you could try using your own qemu built from
source (qemu is very easy to build from source), with a 'qemu
wrapper'.  Qemu wrappers are described in the guestfs(3) manpage.


Note on using KVM
----------------------------------------------------------------------

By default the configure script will look for qemu-kvm (KVM support).
You will need a reasonably recent processor for this to work.  KVM is
much faster than using plain Qemu.

You may also need to enable KVM support for non-root users, by following
these instructions:

  http://www.linux-kvm.org/page/FAQ#How_can_I_use_kvm_with_a_non-privileged_user.3F

On some systems, this will work too:

  chmod o+rw /dev/kvm

On some systems, the chmod will not survive a reboot, and you will
need to make edits to the udev configuration.


vmchannel
----------------------------------------------------------------------

Previous versions of libguestfs required something called "vmchannel".
Vmchannel is a special device given to virtual machines which allows
them to communicate in some way with the host, often (but not always)
without using a traditional network device.  In reality, there is no
one thing called "vmchannel".  This idea has been reimplemented
several times under the name vmchannel, and other hypervisors have
their own incompatible implementation(s) too.

In libguestfs <= 1.0.71, we required a specific vmchannel which is
properly known as "guestfwd" and has been upstream in qemu since here:

  http://lists.gnu.org/archive/html/qemu-devel/2009-02/msg01042.html

In libguestfs >= 1.0.71 we don't require any vmchannel implementation,
as long as qemu has been compiled with support for SLIRP (user mode
networking, or "-net user"), which is almost always the case.

However we still offer the ability to use vmchannel, and in future we
may add support for other types of qemu, which is useful in a few
cases, specifically where qemu packagers decide to compile out support
for SLIRP (qemu packagers: please don't do this).


Supermin appliance
----------------------------------------------------------------------

If you configure with --enable-supermin then we will build a supermin
appliance (supermin = super-minimized).  This is a very specialized
appliance which is built on-the-fly at runtime (specifically, when you
call guestfs_launch).

The normal appliance is a self-contained Linux operating system, based
on the Fedora/RHEL/CentOS Linux distro.  So it contains a complete
copy of all the libraries and programs needed, like kernel, libc,
bash, coreutils etc etc.

The supermin appliance removes the kernel and all the executable
libraries and programs from the appliance.  That just leaves a
skeleton of config files and some data files, which is obviously
massively smaller than the normal appliance.  At runtime we rebuild
the appliance on-the-fly from the libraries and programs on the host
(eg. pulling in the real /lib/libc.so, the real /bin/bash etc.)

Although this process of rebuilding the appliance each time sounds
slow, it turns out to be faster than using the prebuilt appliance.
(Most of the saving comes from not compressing the appliance - it
transpires that decompressing the appliance is the slowest part of the
whole boot sequence).  On my machine, a new appliance can be built in
under a fifth of a second, and the boot time is several seconds
shorter.

The big advantage of the supermin appliance for distributions like
Fedora is that it gets security fixes automatically from the host, so
there is no need to rebuild the whole of libguestfs for a security
update in some underlying library.

There are several DISADVANTAGES:

It won't work at all except in very narrow, controlled cases like the
Fedora packaging case.  We control the dependencies of the libguestfs
RPM tightly to ensure that the required binaries are actually present
on the host.

Furthermore there are certain unlikely changes in the packages on the
host which could break a supermin appliance, eg. an updated library
which depends on an additional data file.

Also supermin appliances are subjected to changes in the host kernel
which might break compatibility with qemu -- these are, of course,
real bugs in any case.

Lastly, supermin appliances really can't be moved between branches of
distributions (eg. built on Fedora 12 and moved to Fedora 10) because
they are not self-contained and they rely on certain libraries being
around.  You shouldn't do this anyway.

Use supermin appliances with caution.


Notes on cross-architecture support
----------------------------------------------------------------------

At the moment we basically don't support cross-architecture or
32-on-64.  This limits what is possible for some guests.  Filesystem
operations and FUSE will work fine, but running commands in guests may
not be possible.

To enable this requires work for cross-architecture and 32-on-64
support in febootstrap, fakeroot and fakechroot.

The daemon/ directory contains its own configure script.  This is so
that in future we will be able to cross-compile the daemon.


Mirroring tip
----------------------------------------------------------------------

On my machines I can usually rebuild the appliance in around 3
minutes.  If it takes much longer for you, use a local Fedora mirror
or squid.

To use squid to cache yum downloads, read this first:
https://lists.dulug.duke.edu/pipermail/yum/2006-August/009041.html
(In brief, because yum chooses random mirrors each time, squid doesn't
work very well with default yum configuration.  To get around this,
choose a Fedora mirror which is close to you, set this with
'./configure --with-mirror=[...]', and then proxy the whole lot
through squid by setting http_proxy environment variable).

You will also need to substantially increase the squid configuration
limits:
http://fedoraproject.org/wiki/Using_Mock_to_test_package_builds#Using_Squid_to_Speed_Up_Mock_package_downloads


Porting to other Linux distros / non-Linux
----------------------------------------------------------------------

libguestfs itself should be fairly portable to other Linux
distributions.  Non-Linux ports are trickier, but we will accept
patches if they aren't too invasive.

The main porting issues are with the dependencies needed to build the
appliance.  You will need to find or port the following packages
first:

 - fakeroot
 - fakechroot
 - python
 - rpm-python    http://www.rpm.org/
 - yum           http://yum.baseurl.org/
 - febootstrap   http://people.redhat.com/~rjones/febootstrap/


Copyright and license information
----------------------------------------------------------------------

Copyright (C) 2009-2010 Red Hat Inc.

The library is distributed under the LGPLv2+.  The programs are
distributed under the GPLv2+.  Please see the files COPYING and
COPYING.LIB for full license information.