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§ion=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
See also the next question.
Why can't I run fsck on a live filesystem using "guestfish --ro"?
This command will usually not work:
guestfish --ro -a /dev/vg/my_root_fs run : fsck /dev/sda
The reason for this is that qemu creates a snapshot over the original filesystem, but it doesn't create a
strict point-in-time snapshot. Blocks of data on the underlying filesystem are read by qemu at different
times as the fsck operation progresses, with host writes in between. The result is that fsck sees
massive corruption (imaginary, not real!) and fails.
What you have to do is to create a point-in-time snapshot. If it's a logical volume, use an LVM2
snapshot. If the filesystem is located inside something like a btrfs/ZFS file, use a btrfs/ZFS snapshot,
and then run the fsck on the snapshot. In practice you don't need to use libguestfs for this -- just run
"/sbin/fsck" directly.
Creating point-in-time snapshots of host devices and files is outside the scope of libguestfs, although
libguestfs can operate on them once they are created.
What's the difference between guestfish and virt-rescue?
A lot of people are confused by the two superficially similar tools we provide:
$ guestfish --ro -a guest.img
><fs> run
><fs> fsck /dev/sda1
$ virt-rescue --ro guest.img
><rescue> /sbin/fsck /dev/sda1
And the related question which then arises is why you can't type in full shell commands with all the
--options in guestfish (but you can in virt-rescue(1)).
guestfish(1) is a program providing structured access to the guestfs(3) API. It happens to be a nice
interactive shell too, but its primary purpose is structured access from shell scripts. Think of it more
like a language binding, like Python and other bindings, but for shell. The key differentiating factor
of guestfish (and the libguestfs API in general) is the ability to automate changes.
virt-rescue(1) is a free-for-all freeform way to boot the libguestfs appliance and make arbitrary changes
to your VM. It's not structured, you can't automate it, but for making quick ad-hoc fixes to your guests,
it can be quite useful.
But, libguestfs also has a "backdoor" into the appliance allowing you to send arbitrary shell commands.
It's not as flexible as virt-rescue, because you can't interact with the shell commands, but here it is
anyway:
><fs> debug sh "cmd arg1 arg2 ..."
Note that you should not rely on this. It could be removed or changed in future. If your program needs
some operation, please add it to the libguestfs API instead.
What's the deal with "guestfish -i"?
Why does virt-cat only work on a real VM image, but virt-df works on any disk image?
What does "no root device found in this operating system image" mean?
These questions are all related at a fundamental level which may not be immediately obvious.
At the guestfs(3) API level, a "disk image" is just a pile of partitions and filesystems.
In contrast, when the virtual machine boots, it mounts those filesystems into a consistent hierarchy such
as:
/ (/dev/sda2)
|
+-- /boot (/dev/sda1)
|
+-- /home (/dev/vg_external/Homes)
|
+-- /usr (/dev/vg_os/lv_usr)
|
+-- /var (/dev/vg_os/lv_var)
(or drive letters on Windows).
The API first of all sees the disk image at the "pile of filesystems" level. But it also has a way to
inspect the disk image to see if it contains an operating system, and how the disks are mounted when the
operating system boots: "INSPECTION" in guestfs(3).
Users expect some tools (like virt-cat(1)) to work with VM paths:
virt-cat fedora.img /var/log/messages
How does virt-cat know that "/var" is a separate partition? The trick is that virt-cat performs
inspection on the disk image, and uses that to translate the path correctly.
Some tools (including virt-cat(1), virt-edit(1), virt-ls(1)) use inspection to map VM paths. Other
tools, such as virt-df(1) and virt-filesystems(1) operate entirely at the raw "big pile of filesystems"
level of the libguestfs API, and don't use inspection.
guestfish(1) is in an interesting middle ground. If you use the -a and -m command line options, then you
have to tell guestfish exactly how to add disk images and where to mount partitions. This is the raw API
level.
If you use the -i option, libguestfs performs inspection and mounts the filesystems for you.
The error "no root device found in this operating system image" is related to this. It means inspection
was unable to locate an operating system within the disk image you gave it. You might see this from
programs like virt-cat if you try to run them on something which is just a disk image, not a virtual
machine disk image.
What do these "debug*" and "internal-*" functions do?
There are some functions which are used for debugging and internal purposes which are not part of the
stable API.
The "debug*" (or "guestfs_debug*") functions, primarily "guestfs_debug" in guestfs(3) and a handful of
others, are used for debugging libguestfs. Although they are not part of the stable API and thus may
change or be removed at any time, some programs may want to call these while waiting for features to be
added to libguestfs.
The "internal-*" (or "guestfs_internal_*") functions are purely to be used by libguestfs itself. There
is no reason for programs to call them, and programs should not try to use them. Using them will often
cause bad things to happen, as well as not being part of the documented stable API.
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.
SEE ALSO
guestfish(1), guestfs(3), http://libguestfs.org/.
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.
libguestfs-1.24.5 2015-10-14 guestfs-faq(1)