Ubuntu Manpage: guestfs-faq - libguestfs Frequently Asked Questions (FAQ)

Provided by: libguestfs0_1.24.5-1ubuntu0.1_amd64

NAME

       guestfs-faq - libguestfs Frequently Asked Questions (FAQ)

ABOUT LIBGUESTFS

What is libguestfs?
libguestfs is a way to create, access and modify disk images. You can look inside disk
images, modify the files they contain, create them from scratch, resize them, and much
more. It's especially useful from scripts and programs and from the command line.

libguestfs is a C library (hence "lib-"), and a set of tools built on this library, and
bindings for many common programming languages.

For more information about what libguestfs can do read the introduction on the home page
(http://libguestfs.org).

What are the virt tools?
Virt tools (website: http://virt-tools.org) are a whole set of virtualization management
tools aimed at system administrators. Some of them come from libguestfs, some from
libvirt and many others from other open source projects. So virt tools is a superset of
libguestfs. However libguestfs comes with many important tools. See
http://libguestfs.org for a full list.

Does libguestfs need { libvirt / KVM / Red Hat / Fedora }?
No!

libvirt is not a requirement for libguestfs.

libguestfs works with any disk image, including ones created in VMware, KVM, qemu,
VirtualBox, Xen, and many other hypervisors, and ones which you have created from scratch.

Red Hat sponsors (ie. pays for) development of libguestfs and a huge number of other open
source projects. But you can run libguestfs and the virt tools on many different Linux
distros and Mac OS X. We try our best to support all Linux distros as first-class
citizens. Some virt tools have been ported to Windows.

How does libguestfs compare to other tools?
vs. kpartx
Libguestfs takes a different approach from kpartx. kpartx needs root, and mounts
filesystems on the host kernel (which can be insecure - see "SECURITY" in guestfs(3)).
Libguestfs isolates your host kernel from guests, is more flexible, scriptable,
supports LVM, doesn't require root, is isolated from other processes, and cleans up
after itself. Libguestfs is more than just file access because you can use it to
create images from scratch.

vs. vdfuse
vdfuse is like kpartx but for VirtualBox images. See the kpartx comparison above.
You can use libguestfs on the partition files exposed by vdfuse, although it's not
necessary since libguestfs can access VirtualBox images directly.

vs. qemu-nbd
NBD (Network Block Device) is a protocol for exporting block devices over the network.
qemu-nbd is an NBD server which can handle any disk format supported by qemu (eg. raw,
qcow2). You can use libguestfs and qemu-nbd together to access block devices over the
network, for example: "guestfish -a nbd://remote"

vs. mounting filesystems in the host
Mounting guest filesystems in the host is insecure and should be avoided completely
for untrusted guests. Use libguestfs to provide a layer of protection against
filesystem exploits. See also guestmount(1).

vs. parted
Libguestfs supports LVM. Libguestfs uses parted and provides most parted features
through the libguestfs API.

GETTING HELP AND REPORTING BUGS

   How do I know what version I'm using?
       The simplest method is:

        guestfish --version

       Libguestfs development happens along an unstable branch and we periodically create a
       stable branch which we backport stable patches to.  To find out more, read "LIBGUESTFS
       VERSION NUMBERS" in guestfs(3).

   How can I get help?
   What mailing lists or chat rooms are available?
       If you are a Red Hat customer using Red Hat Enterprise Linux, please contact
       Red Hat Support: http://redhat.com/support

       There is a mailing list, mainly for development, but users are also welcome to ask
       questions about libguestfs and the virt tools:
       https://www.redhat.com/mailman/listinfo/libguestfs

       You can also talk to us on IRC channel "#libguestfs" on FreeNode.  We're not always
       around, so please stay in the channel after asking your question and someone will get back
       to you.

       For other virt tools (not ones supplied with libguestfs) there is a general virt tools
       mailing list: https://www.redhat.com/mailman/listinfo/virt-tools-list

   How do I report bugs?
       Please use the following link to enter a bug in Bugzilla:

       https://bugzilla.redhat.com/enter_bug.cgi?component=libguestfs&product=Virtualization+Tools

       Include as much detail as you can and a way to reproduce the problem.

       Include the full output of libguestfs-test-tool(1).

COMMON PROBLEMS

See also "LIBGUESTFS GOTCHAS" in guestfs(3) for some "gotchas" with using the libguestfs
API.

"Could not allocate dynamic translator buffer"
This obscure error is in fact an SELinux failure. You have to enable the following
SELinux boolean:

setsebool -P virt_use_execmem=on

For more information see https://bugzilla.redhat.com/show_bug.cgi?id=806106.

"child process died unexpectedly"
[This error message was changed in libguestfs 1.21.18 to something more explanatory.]

This error indicates that qemu failed or the host kernel could not boot. To get further
information about the failure, you have to run:

libguestfs-test-tool

If, after using this, you still don't understand the failure, contact us (see previous
section).

libguestfs: error: cannot find any suitable libguestfs supermin, fixed or old-style appliance
on LIBGUESTFS_PATH
If you see this error on Debian/Ubuntu, you need to run the following command as root:

update-guestfs-appliance

execl: /init: Permission denied
Note: If this error happens when you are using a distro package of libguestfs (eg. from
Fedora, Debian, etc) then file a bug against the distro. This is not an error which
normal users should ever see if the distro package has been prepared correctly.

This error happens during the supermin boot phase of starting the appliance:

supermin: mounting new root on /root
supermin: chroot
execl: /init: Permission denied
supermin: debug: listing directory /
[...followed by a lot of debug output...]

This is a complicated bug related to supermin(1) appliances. The appliance is constructed
by copying files like "/bin/bash" and many libraries from the host. The file "hostfiles"
lists the files that should be copied from the host into the appliance. If some files
don't exist on the host then they are missed out, but if these files are needed in order
to (eg) run "/bin/bash" then you'll see the above error.

Diagnosing the problem involves studying the libraries needed by "/bin/bash", ie:

ldd /bin/bash

comparing that with "hostfiles", with the files actually available in the host filesystem,
and with the debug output printed in the error message. Once you've worked out which file
is missing, install that file using your package manager and try again.

You should also check that files like "/init" and "/bin/bash" (in the appliance) are
executable. The debug output shows file modes.

Non-ASCII characters don't appear on VFAT filesystems.
Typical symptoms of this problem:

• You get an error when you create a file where the filename contains non-ASCII
characters, particularly non 8-bit characters from Asian languages (Chinese, Japanese,
etc). The filesystem is VFAT.

• When you list a directory from a VFAT filesystem, filenames appear as question marks.

This is a design flaw of the GNU/Linux system.

VFAT stores long filenames as UTF-16 characters. When opening or returning filenames, the
Linux kernel has to translate these to some form of 8 bit string. UTF-8 would be the
obvious choice, except for Linux users who persist in using non-UTF-8 locales (the user's
locale is not known to the kernel because it's a function of libc).

Therefore you have to tell the kernel what translation you want done when you mount the
filesystem. The two methods are the "iocharset" parameter (which is not relevant to
libguestfs) and the "utf8" flag.

So to use a VFAT filesystem you must add the "utf8" flag when mounting. From guestfish,
use:

><fs> mount-options utf8 /dev/sda1 /

or on the guestfish command line:

guestfish [...] -m /dev/sda1:/:utf8

or from the API:

guestfs_mount_options (g, "utf8", "/dev/sda1", "/");

The kernel will then translate filenames to and from UTF-8 strings.

We considered adding this mount option transparently, but unfortunately there are several
problems with doing that:

• On some Linux systems, the "utf8" mount option doesn't work. We don't precisely
understand what systems or why, but this was reliably reported by one user.

• It would prevent you from using the "iocharset" parameter because it is incompatible
with "utf8". It is probably not a good idea to use this parameter, but we don't want
to prevent it.

Non-ASCII characters appear as underscore (_) on ISO9660 filesystems.
The filesystem was not prepared correctly with mkisofs or genisoimage. Make sure the
filesystem was created using Joliet and/or Rock Ridge extensions. libguestfs does not
require any special mount options to handle the filesystem.

DOWNLOADING, INSTALLING, COMPILING LIBGUESTFS

Where can I get the latest binaries for ...?
Fedora ≥ 11
Use:

yum install '*guestf*'

For the latest builds, see:
http://koji.fedoraproject.org/koji/packageinfo?packageID=8391

Red Hat Enterprise Linux
RHEL 5
The version shipped in official RHEL 5 is very old and should not be used except
in conjunction with virt-v2v. Use the up-to-date libguestfs 1.20 package in EPEL
5: https://fedoraproject.org/wiki/EPEL

RHEL 6
It is part of the default install. On RHEL 6 and 7 (only) you have to install
"libguestfs-winsupport" to get Windows guest support.

RHEL 6.5
There is a preview repository available for people who want to see what
packages will be in RHEL 6.5. Follow the instructions here:

https://www.redhat.com/archives/libguestfs/2013-May/msg00100.html

RHEL 7
It will be part of the default install, and based on libguestfs 1.22. You will
need to install "libguestfs-winsupport" separately to get Windows guest support.

Debian and Ubuntu
After installing libguestfs you need to do:

sudo update-guestfs-appliance

On Ubuntu only:

sudo chmod 0644 /boot/vmlinuz*

You may need to add yourself to the "kvm" group:

sudo usermod -a -G kvm yourlogin

Debian Squeeze (6)
Hilko Bengen has built libguestfs in squeeze backports:
http://packages.debian.org/search?keywords=guestfs&searchon=names&section=all&suite=squeeze-backports

Debian Wheezy and later (7+)
Hilko Bengen supports libguestfs on Debian. Official Debian packages are
available: http://packages.debian.org/search?keywords=libguestfs

Ubuntu
We don't have a full time Ubuntu maintainer, and the packages supplied by
Canonical (which are outside our control) are sometimes broken.

Canonical decided to change the permissions on the kernel so that it's not
readable except by root. This is completely stupid, but they won't change it
(https://bugs.launchpad.net/ubuntu/+source/linux/+bug/759725). So every user
should do this:

sudo chmod 0644 /boot/vmlinuz*

Ubuntu 12.04
libguestfs in this version of Ubuntu works, but you need to update febootstrap
and seabios to the latest versions.

You need febootstrap ≥ 3.14-2 from:
http://packages.ubuntu.com/precise/febootstrap

After installing or updating febootstrap, rebuild the appliance:

sudo update-guestfs-appliance

You need seabios ≥ 0.6.2-0ubuntu2.1 or ≥ 0.6.2-0ubuntu3 from:
http://packages.ubuntu.com/precise-updates/seabios or
http://packages.ubuntu.com/quantal/seabios

Also you need to do (see above):

sudo chmod 0644 /boot/vmlinuz*

Gentoo
Libguestfs was added to Gentoo in 2012-07 by Andreis Vinogradovs (libguestfs) and
Maxim Koltsov (mainly hivex). Do:

emerge libguestfs

SuSE
Libguestfs was added to SuSE in 2012 by Olaf Hering.

ArchLinux
Libguestfs was added to the AUR in 2010.

Other Linux distro
Compile from source (next section).

Other non-Linux distro
You'll have to compile from source, and port it.

How can I compile and install libguestfs from source?
If your Linux distro has a working port of supermin (that is, Fedora,
Red Hat Enterprise Linux ≥ 6.3, Debian, Ubuntu and ArchLinux) then you should just be able
to compile from source in the usual way. Download the latest tarball from
http://libguestfs.org/download, unpack it, and start by reading the README file.

If you don't have supermin, you will need to use the "fixed appliance method". See:
http://libguestfs.org/download/binaries/appliance/

Patches to port supermin to more Linux distros are welcome.

Why do I get an error when I try to rebuild from the source RPMs supplied by Red Hat / Fedora?
Note: This issue is fixed in Fedora ≥ 19 and RHEL ≥ 6.5.

Because of the complexity of building the libguestfs appliance, the source RPMs provided
cannot be rebuilt directly using "rpmbuild" or "mock".

If you use Koji (which is open source software and may be installed locally), then the
SRPMs can be rebuilt in Koji. https://fedoraproject.org/wiki/Koji

If you don't have or want to use Koji, then you have to give libguestfs access to the
network so it can download the RPMs for building the appliance. You also need to set an
RPM macro to tell libguestfs to use the network. Put the following line into a file
called "$HOME/.rpmmacros":

%libguestfs_buildnet 1

If you are using mock, do:

mock -D '%libguestfs_buildnet 1' [etc]

How can I add support for sVirt?
Note for Fedora/RHEL users: This configuration is the default starting with Fedora 18 and
RHEL 7. If you find any problems, please let us know or file a bug.

SVirt provides a hardened appliance using SELinux, making it very hard for a rogue disk
image to "escape" from the confinement of libguestfs and damage the host (it's fair to say
that even in standard libguestfs this would be hard, but sVirt provides an extra layer of
protection for the host and more importantly protects virtual machines on the same host
from each other).

Currently to enable sVirt you will need libvirt ≥ 0.10.2 (1.0 or later preferred),
libguestfs ≥ 1.20, and the SELinux policies from recent Fedora. If you are not running
Fedora 18+, you will need to make changes to your SELinux policy - contact us on the
mailing list.

Once you have the requirements, do:

./configure --with-default-backend=libvirt # libguestfs >= 1.22
./configure --with-default-attach-method=libvirt # libguestfs <= 1.20
make

Set SELinux to Enforcing mode, and sVirt should be used automatically.

All, or almost all, features of libguestfs should work under sVirt. There is one known
shortcoming: virt-rescue(1) will not use libvirt (hence sVirt), but falls back to direct
launch of qemu. So you won't currently get the benefit of sVirt protection when using
virt-rescue.

You can check if sVirt is being used by enabling libvirtd logging (see
"/etc/libvirt/libvirtd.log"), killing and restarting libvirtd, and checking the log files
for "Setting SELinux context on ..." messages.

In theory sVirt should support AppArmor, but we have not tried it. It will almost
certainly require patching libvirt and writing an AppArmor policy.

Libguestfs has a really long list of dependencies!
That's because it does a lot of things.

Libguestfs -- as it is packaged for Fedora -- satisfies the following conditions:

1. The Fedora package is full featured, that is, it supports every possible feature of
libguestfs (every filesystem, every filesystem tool, etc.)

A common request is to split up libguestfs into separate feature areas so you could,
say, install XFS support and NTFS support separately. This is not possible right now.

2. The download size of the libguestfs package is relatively small (ie. not ten's of
megabytes as it would be if it included a complete, "statically linked" appliance).

3. The Fedora package automatically updates itself if there is a security update. It
doesn't include a huge static blob that has to be rebuilt and users have to re-
download if there is an update.

4. Able to be installed without needing direct network access. This is important when
using closed networks, privately mirrored repositories or RHN Satellite.

5. The Fedora package can be tested during the build.

If you want to drop any one of those conditions, then you can package libguestfs
differently and make it have fewer dependencies, fewer features or a faster start up time:

1. (full featured)
Take "appliance/packagelist.in" in the source, and comment out any features you don't
actually care about. For example if you never anticipate editing a Windows guest,
remove all the ntfs-related packages. You can get away with fewer dependencies.

2. (download size) / 3. (updates)
Use libguestfs-make-fixed-appliance(1) to build a compressed appliance. Bundle this
with your package and set $LIBGUESTFS_PATH to point to it. Users will have to
download this large appliance, but no dependencies are needed, and supermin-helper(1)
is not used.

4. (network access)
Reconstruct and cache the appliance once during package install. The Debian packaging
currently works like this, but requires network access during package install.

5. (tests)
Don't run any tests during the build. The build will be much faster, but also less
likely to work correctly.

Note that running the tests in "tests/qemu" is probably a good idea, since those are
sanity tests. Also you should do "make quickcheck" to ensure libguestfs is basically
working.

Errors during launch on Fedora ≥ 18, RHEL ≥ 7
In Fedora ≥ 18 and RHEL ≥ 7, libguestfs uses libvirt to manage the appliance. Previously
(and upstream) libguestfs runs qemu directly:

+----------------------------------+
| libguestfs |
+----------------+-----------------+
| direct backend | libvirt backend |
+----------------+-----------------+
| |
v v
+-------+ +----------+
| qemu | | libvirtd |
+-------+ +----------+
|
v
+-------+
| qemu |
+-------+

upstream Fedora 18+
non-Fedora RHEL 7+
non-RHEL

The libvirt backend is more sophisticated, supporting SELinux/sVirt (see above),
hotplugging and more. It is, however, more complex and so less robust.

If you have permissions problems using the libvirt backend, you can switch to the direct
backend by setting this environment variable:

export LIBGUESTFS_BACKEND=direct

before running any libguestfs program or virt tool.

How can I switch to a fixed / prebuilt appliance?
This may improve the stability and performance of libguestfs on Fedora and RHEL.

Any time after installing libguestfs, run the following commands as root:

mkdir -p /usr/local/lib/guestfs/appliance
libguestfs-make-fixed-appliance /usr/local/lib/guestfs/appliance
ls -l /usr/local/lib/guestfs/appliance

Now set the following environment variable before using libguestfs or any virt tool:

export LIBGUESTFS_PATH=/usr/local/lib/guestfs/appliance

Of course you can change the path to any directory you want. You can share the appliance
across machines that have the same architecture (eg. all x86-64), but note that libvirt
will prevent you from sharing the appliance across NFS because of permissions problems (so
either switch to the direct backend or don't use NFS).

How can I speed up libguestfs builds?
By far the most important thing you can do is to install and properly configure Squid.
Note that the default configuration that ships with Squid is rubbish, so configuring it is
not optional.

A very good place to start with Squid configuration is here:
https://fedoraproject.org/wiki/Extras/MockTricks#Using_Squid_to_Speed_Up_Mock_package_downloads

Make sure Squid is running, and that the environment variables $http_proxy and $ftp_proxy
are pointing to it.

With Squid running and correctly configured, appliance builds should be reduced to a few
minutes.

How can I speed up libguestfs builds (Debian)?

Hilko Bengen suggests using "approx" which is a Debian archive proxy
(http://packages.debian.org/approx). This tool is documented on Debian in the approx(8)
manual page.

SPEED, DISK SPACE USED BY LIBGUESTFS

Note: Most of the information in this section has moved: guestfs-performance(1).

Upload or write seem very slow.
In libguestfs < 1.13.16, the mount command ("guestfs_mount" in guestfs(3)) enabled option
"-o sync" implicitly. This causes very poor write performance, and was one of the main
gotchas for new libguestfs users.

For libguestfs < 1.13.16, replace mount with "mount-options", leaving the first parameter
as an empty string.

You can also do this with more recent versions of libguestfs, but if you know that you are
using libguestfs ≥ 1.13.16 then it's safe to use plain mount.

If the underlying disk is not fully allocated (eg. sparse raw or qcow2) then writes can be
slow because the host operating system has to do costly disk allocations while you are
writing. The solution is to use a fully allocated format instead, ie. non-sparse raw, or
qcow2 with the "preallocation=metadata" option.

Libguestfs uses too much disk space!
libguestfs caches a large-ish appliance in:

/var/tmp/.guestfs-<UID>

If the environment variable "TMPDIR" is defined, then "$TMPDIR/.guestfs-<UID>" is used
instead.

It is safe to delete this directory when you are not using libguestfs.

virt-sparsify seems to make the image grow to the full size of the virtual disk
If the input to virt-sparsify(1) is raw, then the output will be raw sparse. Make sure
you are measuring the output with a tool which understands sparseness such as "du -sh".
It can make a huge difference:

$ ls -lh test1.img
-rw-rw-r--. 1 rjones rjones 100M Aug 8 08:08 test1.img
$ du -sh test1.img
3.6M test1.img

(Compare the apparent size 100M vs the actual size 3.6M)

If all this confuses you, use a non-sparse output format by specifying the --convert
option, eg:

virt-sparsify --convert qcow2 disk.raw disk.qcow2

Why doesn't virt-resize work on the disk image in-place?
Resizing a disk image is very tricky -- especially making sure that you don't lose data or
break the bootloader. The current method effectively creates a new disk image and copies
the data plus bootloader from the old one. If something goes wrong, you can always go
back to the original.

If we were to make virt-resize work in-place then there would have to be limitations: for
example, you wouldn't be allowed to move existing partitions (because moving data across
the same disk is most likely to corrupt data in the event of a power failure or crash),
and LVM would be very difficult to support (because of the almost arbitrary mapping
between LV content and underlying disk blocks).

Another method we have considered is to place a snapshot over the original disk image, so
that the original data is untouched and only differences are recorded in the snapshot.
You can do this today using "qemu-img create" + "virt-resize", but qemu currently isn't
smart enough to recognize when the same block is written back to the snapshot as already
exists in the backing disk, so you will find that this doesn't save you any space or time.

In summary, this is a hard problem, and what we have now mostly works so we are reluctant
to change it.

Why doesn't virt-sparsify work on the disk image in-place?
Eventually we plan to make virt-sparsify work on disk images in-place, instead of copying
the disk image. However it requires several changes to both the Linux kernel and qemu
which are slowly making their way upstream (thanks to the tireless efforts of Paolo
Bonzini). Then we will have to modify virt-sparsify to support this. Finally there will
be some integration work required to make sure all the pieces work together.

Even with this implemented there may be some limitations: For example, it requires
completely different steps (and is probably harder) to sparsify a disk image that is
stored on a SAN LUN, compared to one which is stored in a local raw image file, so you can
expect that different storage and backing formats will become supported at different
times. Some backing filesystems / formats may never support sparsification (eg. disk
images stored on VFAT, old-style non-thin LVs).

USING LIBGUESTFS IN YOUR OWN PROGRAMS

   The API has hundreds of methods, where do I start?
       We recommend you start by reading the API overview: "API OVERVIEW" in guestfs(3).

       Although the API overview covers the C API, it is still worth reading even if you are
       going to use another programming language, because the API is the same, just with simple
       logical changes to the names of the calls:

                         C  guestfs_ln_sf (g, target, linkname);
                    Python  g.ln_sf (target, linkname);
                     OCaml  g#ln_sf target linkname;
                      Perl  $g->ln_sf (target, linkname);
         Shell (guestfish)  ln-sf target linkname
                       PHP  guestfs_ln_sf ($g, $target, $linkname);

       Once you're familiar with the API overview, you should look at this list of starting
       points for other language bindings: "USING LIBGUESTFS WITH OTHER PROGRAMMING LANGUAGES" in
       guestfs(3).

   Can I use libguestfs in my proprietary / closed source / commercial program?
       In general, yes.  However this is not legal advice - read the license that comes with
       libguestfs, and if you have specific questions contact a lawyer.

       In the source tree the license is in the file "COPYING.LIB" (LGPLv2+ for the library and
       bindings) and "COPYING" (GPLv2+ for the standalone programs).

DEBUGGING LIBGUESTFS

Help, it's not working!
Please supply all the information in this checklist, in an email sent to "libguestfs" @
"redhat.com":

• What are you trying to achieve?

• What exact commands did you run?

• What was the precise error / output of these commands?

• Enable debugging, run the commands again, and capture the complete output. Do not
edit the output.

export LIBGUESTFS_DEBUG=1
export LIBGUESTFS_TRACE=1

• Include the version of libguestfs, the operating system version, and how you installed
libguestfs (eg. from source, "yum install", etc.)

• If no libguestfs program seems to work at all, run the program below and paste the
complete, unedited output into the email:

libguestfs-test-tool

How do I debug when using any libguestfs program or tool (eg. virt-v2v or virt-df)?
There are two "LIBGUESTFS_*" environment variables you can set in order to get more
information from libguestfs.

"LIBGUESTFS_TRACE"
Set this to 1 and libguestfs will print out each command / API call in a format which
is similar to guestfish commands.

"LIBGUESTFS_DEBUG"
Set this to 1 in order to enable massive amounts of debug messages. If you think
there is some problem inside the libguestfs appliance, then you should use this
option.

To set these from the shell, do this before running the program:

export LIBGUESTFS_TRACE=1
export LIBGUESTFS_DEBUG=1

For csh/tcsh the equivalent commands would be:

setenv LIBGUESTFS_TRACE 1
setenv LIBGUESTFS_DEBUG 1

For further information, see: "ENVIRONMENT VARIABLES" in guestfs(3).

How do I debug when using guestfish?
You can use the same environment variables above. Alternatively use the guestfish options
-x (to trace commands) or -v (to get the full debug output), or both.

For further information, see: guestfish(1).

How do I debug when using the API?
Call "guestfs_set_trace" in guestfs(3) to enable command traces, and/or
"guestfs_set_verbose" in guestfs(3) to enable debug messages.

For best results, call these functions as early as possible, just after creating the
guestfs handle if you can, and definitely before calling launch.

How do I capture debug output and put it into my logging system?
Use the event API. For examples, see: "SETTING CALLBACKS TO HANDLE EVENTS" in guestfs(3)
and the "examples/debug-logging.c" program in the libguestfs sources.

Digging deeper into the appliance boot process.
Enable debugging and then read this documentation on the appliance boot process:
"INTERNALS" in guestfs(3).

libguestfs hangs or fails during run/launch.
Enable debugging and look at the full output. If you cannot work out what is going on,
file a bug report, including the complete output of libguestfs-test-tool(1).

DESIGN/INTERNALS OF LIBGUESTFS

Why don't you do everything through the FUSE / filesystem interface?
We offer a command called guestmount(1) which lets you mount guest filesystems on the
host. This is implemented as a FUSE module. Why don't we just implement the whole of
libguestfs using this mechanism, instead of having the large and rather complicated API?

The reasons are twofold. Firstly, libguestfs offers API calls for doing things like
creating and deleting partitions and logical volumes, which don't fit into a filesystem
model very easily. Or rather, you could fit them in: for example, creating a partition
could be mapped to "mkdir /fs/hda1" but then you'd have to specify some method to choose
the size of the partition (maybe "echo 100M > /fs/hda1/.size"), and the partition type,
start and end sectors etc., but once you've done that the filesystem-based API starts to
look more complicated than the call-based API we currently have.

The second reason is for efficiency. FUSE itself is reasonably efficient, but it does
make lots of small, independent calls into the FUSE module. In guestmount these have to
be translated into messages to the libguestfs appliance which has a big overhead (in time
and round trips). For example, reading a file in 64 KB chunks is inefficient because each
chunk would turn into a single round trip. In the libguestfs API it is much more
efficient to download an entire file or directory through one of the streaming calls like
"guestfs_download" or "guestfs_tar_out".

Why don't you do everything through GVFS?
The problems are similar to the problems with FUSE.

GVFS is a better abstraction than POSIX/FUSE. There is an FTP backend for GVFS, which is
encouraging because FTP is conceptually similar to the libguestfs API. However the GVFS
FTP backend makes multiple simultaneous connections in order to keep interactivity, which
we can't easily do with libguestfs.

Why can I write to the disk, even though I added it read-only?
Why does "--ro" appear to have no effect?
When you add a disk read-only, libguestfs places a writable overlay on top of the
underlying disk. Writes go into this overlay, and are discarded when the handle is closed
(or "guestfish" etc. exits).

There are two reasons for doing it this way: Firstly read-only disks aren't possible in
many cases (eg. IDE simply doesn't support them, so you couldn't have an IDE-emulated
read-only disk, although this is not common in real libguestfs installations).

Secondly and more importantly, even if read-only disks were possible, you wouldn't want
them. Mounting any filesystem that has a journal, even "mount -o ro", causes writes to
the filesystem because the journal has to be replayed and metadata updated. If the disk
was truly read-only, you wouldn't be able to mount a dirty filesystem.

To make it usable, we create the overlay as a place to temporarily store these writes, and
then we discard it afterwards. This ensures that the underlying disk is always untouched.

Note also that there is a regression test for this when building libguestfs (in
"tests/qemu"). This is one reason why it's important for packagers to run the test suite.

Does "--ro" make all disks read-only?
No! The "--ro" option only affects disks added on the command line, ie. using "-a" and
"-d" options.

In guestfish, if you use the "add" command, then disk is added read-write (unless you
specify the "readonly:true" flag explicitly with the command).

Can I use "guestfish --ro" as a way to backup my virtual machines?
Usually this is not a good idea. The question is answered in more detail in this mailing
list posting: https://www.redhat.com/archives/libguestfs/2010-August/msg00024.html

DEVELOPERS

Where do I send patches?
Please send patches to the libguestfs mailing list
https://www.redhat.com/mailman/listinfo/libguestfs. You don't have to be subscribed, but
there will be a delay until your posting is manually approved.

Please don't use github pull requests - they will be ignored. The reasons are (a) we want
to discuss and dissect patches on the mailing list, and (b) github pull requests turn into
merge commits but we prefer to have a linear history.

How do I propose a feature?
Large new features that you intend to contribute should be discussed on the mailing list
first (https://www.redhat.com/mailman/listinfo/libguestfs). This avoids disappointment
and wasted work if we don't think the feature would fit into the libguestfs project.

If you want to suggest a useful feature but don't want to write the code, you can file a
bug (see "GETTING HELP AND REPORTING BUGS") with "RFE: " at the beginning of the Summary
line.

Who can commit to libguestfs git?
About 5 people have commit access to github. Patches should be posted on the list first
and ACKed. The policy for ACKing and pushing patches is outlined here:

https://www.redhat.com/archives/libguestfs/2012-January/msg00023.html

Can I fork libguestfs?
Of course you can. Git makes it easy to fork libguestfs. Github makes it even easier.
It's nice if you tell us on the mailing list about forks and the reasons for them.

AUTHORS

       Richard W.M. Jones ("rjones at redhat dot com")

COPYRIGHT

       Copyright (C) 2012-2014 Red Hat Inc.

LICENSE

       This library is free software; you can redistribute it and/or modify it under the terms of
       the GNU Lesser General Public License as published by the Free Software Foundation; either
       version 2 of the License, or (at your option) any later version.

       This library is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY;
       without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
       See the GNU Lesser General Public License for more details.

       You should have received a copy of the GNU Lesser General Public License along with this
       library; if not, write to the Free Software Foundation, Inc., 51 Franklin Street, Fifth
       Floor, Boston, MA 02110-1301 USA

BUGS

       To get a list of bugs against libguestfs, use this link:
       https://bugzilla.redhat.com/buglist.cgi?component=libguestfs&product=Virtualization+Tools

       To report a new bug against libguestfs, use this link:
       https://bugzilla.redhat.com/enter_bug.cgi?component=libguestfs&product=Virtualization+Tools

       When reporting a bug, please supply:

       •   The version of libguestfs.

       •   Where you got libguestfs (eg. which Linux distro, compiled from source, etc)

       •   Describe the bug accurately and give a way to reproduce it.

       •   Run libguestfs-test-tool(1) and paste the complete, unedited output into the bug
           report.

NAME

ABOUT LIBGUESTFS

GETTING HELP AND REPORTING BUGS

COMMON PROBLEMS

DOWNLOADING, INSTALLING, COMPILING LIBGUESTFS

SPEED, DISK SPACE USED BY LIBGUESTFS

USING LIBGUESTFS IN YOUR OWN PROGRAMS

DEBUGGING LIBGUESTFS

DESIGN/INTERNALS OF LIBGUESTFS

DEVELOPERS

SEE ALSO

AUTHORS

COPYRIGHT

LICENSE

BUGS