Thank you, Rich,
This was the issue indeed. export LIBGUESTFS_BACKEND=direct fixed it.

The next step I tried was to integrate libguestfs in collectd virt plugin to collect this data automatically.
In this case I'm having an unknown error in add_libvirt_dom() (same with add_domain) when it's invoking qemu-img to create overlay image.

There is no difference between manual and service execution.
I tried setting LIBGUESTFS_BACKEND to direct, libvirt, libvirt:qemu:///session with no success.
Also tried using a different tmp dir just in case - nothing.

Maybe something is wrong with how collectd runs its plugins (dynamic linking)?
Invoking virt-df from collectd's plugin gives the same error message.
I tried running the same qemu-img command from collectd and it passes though! Confusing...

Do you have any hints what to try?

Here is the complete output from collectd:
[2018-11-07 12:21:54] plugin_load: plugin "logfile" successfully loaded.
[2018-11-07 12:21:54] plugin_load: plugin "syslog" successfully loaded.
Before guestfs_create()
UID: 0
Effective UID: 0
Before guestfs_set_verbose()
Before guestfs_set_trace()
Before guestfs_add_domain()
libguestfs: trace: add_domain "wer" "readonly:true"
libguestfs: opening libvirt handle: URI = NULL, auth = default+wrapper, flags = 0
libguestfs: successfully opened libvirt handle: conn = 0x7f7e70005210
libguestfs: trace: add_libvirt_dom (virDomainPtr)0x7f7e70005fa0 "readonly:true"
libguestfs: original domain XML:\n<domain type='kvm' id='9'>\n  <name>wer</name>\n  <uuid>03fde334-92a5-4e5e-9247-924c9d40230f</uuid>\n  <memory unit='KiB'>2097152</memory>\n  <currentMemory unit='KiB'>2097152</currentMemory>\n  <vcpu placement='static'>1</vcpu>\n  <resource>\n    <partition>/machine</partition>\n  </resource>\n  <os>\n    <type arch='x86_64' machine='pc-i440fx-2.7'>hvm</type>\n    <bootmenu enable='yes'/>\n  </os>\n  <features>\n    <acpi/>\n    <apic/>\n    <vmport state='off'/>\n  </features>\n  <cpu mode='custom' match='exact'>\n    <model fallback='allow'>Westmere</model>\n  </cpu>\n  <clock offset='utc'>\n    <timer name='rtc' tickpolicy='catchup'/>\n    <timer name='pit' tickpolicy='delay'/>\n    <timer name='hpet' present='no'/>\n  </clock>\n  <on_poweroff>destroy</on_poweroff>\n  <on_reboot>restart</on_reboot>\n  <on_crash>destroy</on_crash>\n  <pm>\n    <suspend-to-mem enabled='no'/>\n    <suspend-to-disk enabled='no'/>\n  </pm>\n  <devices>\n    <emulator>/usr/bin/qemu-kvm</emulator>\n    <disk type='file' device='disk'>\n      <driver name='qemu' type='qcow2'/>\n      <source file='/home/peterd/TVE/wer.qcow2'/>\n      <backingStore/>\n      <target dev='sda' bus='sata'/>\n      <boot order='1'/>\n      <alias name='sata0-0-0'/>\n      <address type='drive' controller='0' bus='0' target='0' unit='0'/>\n    </disk>\n    <disk type='file' device='disk'>\n      <driver name='qemu' type='qcow2'/>\n      <source file='/var/lib/libvirt/images/wer.qcow2'/>\n      <backingStore/>\n      <target dev='sdb' bus='sata'/>\n      <alias name='sata0-0-1'/>\n      <address type='drive' controller='0' bus='0' target='0' unit='1'/>\n    </disk>\n    <disk type='file' device='cdrom'>\n      <driver name='qemu' type='raw'/>\n      <source file='/home/peterd/boot_new.iso'/>\n      <backingStore/>\n      <target dev='hda' bus='ide'/>\n      <readonly/>\n      <boot order='2'/>\n      <alias name='ide0-0-0'/>\n      <address type='drive' controller='0' bus='0' target='0' unit='0'/>\n    </disk>\n    <controller type='usb' index='0' model='ich9-ehci1'>\n      <alias name='usb'/>\n      <address type='pci' domain='0x0000' bus='0x00' slot='0x06' function='0x7'/>\n    </controller>\n    <controller type='usb' index='0' model='ich9-uhci1'>\n      <alias name='usb'/>\n      <master startport='0'/>\n      <address type='pci' domain='0x0000' bus='0x00' slot='0x06' function='0x0' multifunction='on'/>\n    </controller>\n    <controller type='usb' index='0' model='ich9-uhci2'>\n      <alias name='usb'/>\n      <master startport='2'/>\n      <address type='pci' domain='0x0000' bus='0x00' slot='0x06' function='0x1'/>\n    </controller>\n    <controller type='usb' index='0' model='ich9-uhci3'>\n      <alias name='usb'/>\n      <master startport='4'/>\n      <address type='pci' domain='0x0000' bus='0x00' slot='0x06' function='0x2'/>\n    </controller>\n    <controller type='ide' index='0'>\n      <alias name='ide'/>\n      <address type='pci' domain='0x0000' bus='0x00' slot='0x01' function='0x1'/>\n    </controller>\n    <controller type='sata' index='0'>\n      <alias name='sata0'/>\n      <address type='pci' domain='0x0000' bus='0x00' slot='0x05' function='0x0'/>\n    </controller>\n    <controller type='virtio-serial' index='0'>\n      <alias name='virtio-serial0'/>\n      <address type='pci' domain='0x0000' bus='0x00' slot='0x07' function='0x0'/>\n    </controller>\n    <controller type='scsi' index='0' model='virtio-scsi'>\n      <alias name='scsi0'/>\n      <address type='pci' domain='0x0000' bus='0x00' slot='0x09' function='0x0'/>\n    </controller>\n    <controller type='pci' index='0' model='pci-root'>\n      <alias name='pci.0'/>\n    </controller>\n    <interface type='direct'>\n      <mac address='52:54:00:2c:7a:bf'/>\n      <source dev='macvlan0' mode='bridge'/>\n      <target dev='macvtap0'/>\n      <model type='e1000'/>\n      <alias name='net0'/>\n      <address type='pci' domain='0x0000' bus='0x00' slot='0x03' function='0x0'/>\n    </interface>\n    <serial type='pty'>\n      <source path='/dev/pts/8'/>\n      <target port='0'/>\n      <alias name='serial0'/>\n    </serial>\n    <console type='pty' tty='/dev/pts/8'>\n      <source path='/dev/pts/8'/>\n      <target type='serial' port='0'/>\n      <alias name='serial0'/>\n    </console>\n    <channel type='spicevmc'>\n      <target type='virtio' name='com.redhat.spice.0' state='disconnected'/>\n      <alias name='channel0'/>\n      <address type='virtio-serial' controller='0' bus='0' port='1'/>\n    </channel>\n    <input type='mouse' bus='ps2'>\n      <alias name='input0'/>\n    </input>\n    <input type='keyboard' bus='ps2'>\n      <alias name='input1'/>\n    </input>\n    <graphics type='spice' port='5900' autoport='yes' listen=''>\n      <listen type='address' address=''/>\n      <image compression='off'/>\n    </graphics>\n    <sound model='ich6'>\n      <alias name='sound0'/>\n      <address type='pci' domain='0x0000' bus='0x00' slot='0x04' function='0x0'/>\n    </sound>\n    <video>\n      <model type='qxl' ram='65536' vram='65536' vgamem='16384' heads='1' primary='yes'/>\n      <alias name='video0'/>\n      <address type='pci' domain='0x0000' bus='0x00' slot='0x02' function='0x0'/>\n    </video>\n    <redirdev bus='usb' type='spicevmc'>\n      <alias name='redir0'/>\n      <address type='usb' bus='0' port='1'/>\n    </redirdev>\n    <redirdev bus='usb' type='spicevmc'>\n      <alias name='redir1'/>\n      <address type='usb' bus='0' port='2'/>\n    </redirdev>\n    <memballoon model='virtio'>\n      <alias name='balloon0'/>\n      <address type='pci' domain='0x0000' bus='0x00' slot='0x08' function='0x0'/>\n    </memballoon>\n  </devices>\n  <seclabel type='none' model='none'/>\n  <seclabel type='dynamic' model='dac' relabel='yes'>\n    <label>+0:+0</label>\n    <imagelabel>+0:+0</imagelabel>\n  </seclabel>\n</domain>\n
libguestfs: trace: clear_backend_setting "internal_libvirt_norelabel_disks"
libguestfs: trace: clear_backend_setting = 0
libguestfs: disk[0]: filename: /home/peterd/TVE/wer.qcow2
libguestfs: trace: add_drive "/home/peterd/TVE/wer.qcow2" "readonly:true" "format:qcow2"
libguestfs: creating COW overlay to protect original drive content
libguestfs: trace: get_tmpdir
libguestfs: trace: get_tmpdir = "/tmp"
libguestfs: trace: disk_create "/tmp/libguestfsUIZbDK/overlay1.qcow2" "qcow2" -1 "backingfile:/home/peterd/TVE/wer.qcow2" "backingformat:qcow2"
libguestfs: command: run: qemu-img
libguestfs: command: run: \ create
libguestfs: command: run: \ -f qcow2
libguestfs: command: run: \ -o backing_file=/home/peterd/TVE/wer.qcow2,backing_fmt=qcow2
libguestfs: command: run: \ /tmp/libguestfsUIZbDK/overlay1.qcow2
Formatting '/tmp/libguestfsUIZbDK/overlay1.qcow2', fmt=qcow2 size=107374182400 backing_file=/home/peterd/TVE/wer.qcow2 backing_fmt=qcow2 encryption=off cluster_size=65536 lazy_refcounts=off refcount_bits=16
libguestfs: error: command: waitpid: No child processes
libguestfs: error: qemu-img: /tmp/libguestfsUIZbDK/overlay1.qcow2: qemu-img exited for an unknown reason (status -1), see debug messages above
libguestfs: trace: disk_create = -1 (error)
libguestfs: trace: add_drive = -1 (error)
libguestfs: trace: add_libvirt_dom = -1 (error)
libguestfs: trace: add_domain = -1 (error)
libguestfs: trace: close
libguestfs: closing guestfs handle 0x7f7e700049f0 (state 0)
libguestfs: command: run: rm
libguestfs: command: run: \ -rf /tmp/libguestfsUIZbDK
libguestfs: error: command: waitpid: No child processes

Best Regards,

On Fri, Nov 2, 2018 at 9:48 PM Richard W.M. Jones <> wrote:
On Fri, Nov 02, 2018 at 06:04:08PM +0200, Peter Dimitrov wrote:
> Hello,
> I have a simple C program that uses libguestfs to extract info about disk
> usage from a libvirt domain. It works when ran manually as root, but fails
> when started as a systemd service.
> I'm attaching the service file, source code and verbose logs from both the
> successful manual run and from the service journal.
> SELinix is disabled.
> Error messages:
> libguestfs: set_socket_create_context: getcon failed: (none): Invalid
> argument [you can ignore this message if you are not using SELinux + sVirt]
> libguestfs: clear_socket_create_context: setsockcreatecon failed: NULL:
> Invalid argument [you can ignore this message if you are not using SELinux
> + sVirt]
> libguestfs: error: chown: /tmp/libguestfsvMMaec/guestfsd.sock: Operation
> not permitted
> libguestfs: clear_socket_create_context: setsockcreatecon failed: NULL:
> Invalid argument [you can ignore this message if you are not using SELinux
> + sVirt]
> libguestfs: trace: launch = -1 (error)
> failed to launch domain: Invalid argument

I cannot see what the problem is immediately, but I guess that systemd
is confining the service in such a way that libvirt has problems.

Firstly I would try using the direct backend:

  export LIBGUESTFS_BACKEND=direct

If it's still not fixed, then it's something to do with systemd
confining affecting qemu.

If that fixes it, then it's a problem with libvirt, and you will need
to find the libvirt log file.  Usually that's in
/var/log/libvirt/qemu/guestfs-*.log or in
$HOME/.cache/libvirt/qemu/log/guestfs-*.log  But I've no idea where it
will end up when you're running everything under systemd.


Richard Jones, Virtualization Group, Red Hat
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