Most people start nbdkit from the command line or run it from another program (see nbdkit-captive(1). It is also possible to run nbdkit as a standalone service, which is what this page describes.
nbdkit supports socket activation (sometimes called systemd socket activation). This is a simple protocol where instead of nbdkit itself opening the listening socket(s), the parent process (typically systemd) passes in pre-opened file descriptors. Socket activation lets you serve infrequent NBD requests using a superserver without needing nbdkit to be running the whole time.
Socket activation is triggered when both the
LISTEN_PID environment variables are set. In this mode using -i, -p, --run, -s or -U flags on the command line is illegal and will cause an error. Also in this mode nbdkit does not fork into the background (ie. -f is implied).
Using socket activation with systemd
To use nbdkit with socket activation from systemd, create a unit file ending in
.socket (eg. /etc/systemd/system/nbdkit.socket) containing:
[Unit] Description=NBDKit Network Block Device server [Socket] ListenStream=10809 [Install] WantedBy=sockets.target
There are various formats for the
ListenStream key. See systemd.socket(5) for more information.
Also create a service unit (eg. /etc/systemd/system/nbdkit.service) containing:
[Service] ExecStart=/usr/sbin/nbdkit file /path/to/serve
For more information on systemd and socket activation, see http://0pointer.de/blog/projects/socket-activation.html
Error messages from nbdkit can be sent to standard error (--log=stderr), or to the system log (--log=syslog), or can be discarded completely (--log=null, not recommended for normal use).
The default, if --log is not specified on the command line, is to send error messages to stderr, unless nbdkit forks into the background in which case they are sent to syslog.
- Messages go to standard error (stderr):
When running from the command line in the foreground.
When using systemd socket activation.
Using --log=stderr forces all messages to go to standard error.
- Messages go to the system log (syslog):
When running from the command line, forked into the background.
Using --log=syslog forces all messages to go to the system log.
Debug messages (-v/--verbose) always go to standard error and are never sent to the system log.
On Linux nbdkit supports the
AF_VSOCK address family / protocol. This allows you to serve NBD devices into virtual machines without using a regular network connection.
Note that this is different from the usual case where you present NBD as a virtual block device to a guest (which the guest sees as something like a SATA or virtio-scsi disk). With
AF_VSOCK the virtual machine sees a raw NBD socket which it can connect to by opening an
AF_VSOCK connection. Only libnbd supports
AF_VSOCK NBD client connections at the time of writing (2019). For more about this protocol, see https://wiki.qemu.org/Features/VirtioVsock
To set up an
AF_VSOCK server, use for example:
nbdkit --vsock [--port PORT] memory 1G
The optional -p/--port argument is used to change the
AF_VSOCK port number. These port numbers exist in a different namespace from TCP/IP port numbers. Also unlike TCP, the port numbers are 32 bit. The default port is 10809.
The guest that wishes to access nbdkit must be configured for virtio-vsock. On the qemu command line use:
qemu ... -device vhost-vsock-pci,id=vhost-vsock-pci0
For libvirt add this element to the
If you see the error
unable to open vhost-vsock device then you may have to unload the VMCI transport on the host:
modprobe -r vmw_vsock_vmci_transport
Once nbdkit and the guest are running, from inside the guest you can connect to nbdkit on the host using libnbd:
nbdsh -c 'h.connect_vsock(2, 10809)' -c 'print(h.get_size())'
If present in the environment when nbdkit starts up, these trigger “Socket Activation”.
nbdkit(1), nbdkit-client(1), nbdkit-exitlast-filter(1), nbdkit-exitwhen-filter(1), nbdkit-ip-filter(1), nbdkit-limit-filter(1), systemd(1), systemd.socket(5), syslog(3), rsyslogd(8), journalctl(1), nbdsh(1).
Richard W.M. Jones
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nbdkit(1), nbdkit-client(1), nbdkit-exitlast-filter(1), nbdkit-exitwhen-filter(1), nbdkit-nbd-plugin(1), nbdkit-tmpdisk-plugin(1).