Provided by: nbdkit_1.24.1-2ubuntu4_amd64 
NAME
nbdkit-streaming-plugin - nbdkit streaming plugin
SYNOPSIS
nbdkit streaming write=PIPE [size=SIZE]
nbdkit streaming read=PIPE [size=SIZE]
DESCRIPTION
"nbdkit-streaming-plugin" is a plugin for nbdkit(1) that can stream in or out of a local
pipe or socket. An alternate tool which can do something similar to this is nbdcopy(1).
To turn an NBD export into a local file use nbdfuse(1).
If the NBD client opens the NBD port and writes from the start to the end of the disk
without seeking backwards, then you can turn that into a stream of data on a local pipe or
socket using:
nbdkit streaming write=./pipe
This can be visualised as:
┌───────────┐
NBD │ nbdkit │ plugin streams
client ──────▶│ streaming │──────▶ data to ./pipe
writes │ plugin │
└───────────┘
If the NBD client opens the NBD port and reads from the start to the end of the disk
without seeking backwards, then you can turn a local pipe or socket into a stream of data
for that client:
nbdkit streaming read=./pipe
This can be visualised as:
┌───────────┐
plugin streams │ nbdkit │ NBD
data from ──────▶│ streaming │──────▶ client
./pipe │ plugin │ reads
└───────────┘
Note that ./pipe (or the local socket) sees raw data, it is not using the NBD protocol.
If you want to forward NBD to a local socket connected to another NBD server, use
nbdkit-nbd-plugin(1).
Combining this plugin with qemu tools
Typical usage is with qemu tools. The following command does not work because the output
is a pipe or socket:
$ mkfifo pipe
$ qemu-img convert -n input -O raw ./pipe
qemu-img: Could not open './pipe': A regular file was expected by
the 'file' driver, but something else was given
However this will work:
nbdkit -U - streaming write=./pipe \
--run ' qemu-img convert -n input -O raw $nbd '
┌───────────┐ ┌───────────┐
reads │ qemu-img │ │ nbdkit │ plugin streams
input ──────▶│ convert │──────▶│ streaming │──────▶ data to ./pipe
file │ command │ │ plugin │
└───────────┘ ↑└───────────┘
Unix domain socket (-U -)
This only works because the output format is raw and qemu-img can write that linearly
(without seeking backwards or writing blocks out of order). This approach cannot work for
other formats such as qcow2 since those contain metadata that must be updated by seeking
back to the start of the file which is not possible if the output is a pipe.
The reverse is to get qemu-img to read from a pipe:
nbdkit -U - streaming read=./pipe \
--run ' qemu-img convert -f raw $nbd -O qcow2 output.qcow2 '
For use of the --run and -U - options, see nbdkit-captive(1).
PARAMETERS
Either "read" or "write" is required, but not both.
read=FILENAME
(nbdkit ≥ 1.22)
Read data stream from the named pipe or socket. If the pipe or socket does not exist,
then it is created (as a named FIFO), otherwise the existing pipe or socket is opened
and used.
write=FILENAME
Write data stream to the named pipe or socket. If the pipe or socket does not exist,
then it is created (as a named FIFO), otherwise the existing pipe or socket is opened
and used.
pipe=FILENAME
For backwards compatibility with nbdkit ≤ 1.20 this is a synonym for "write=FILENAME".
size=SIZE
Specify the virtual size of the stream.
This parameter is optional. If not specified, then the virtual disk appears to the
client to be very large (effectively infinite). Whether you need to specify this
parameter depends on the client. Some clients don't check the size and just
write/stream, others do checks or calculations based on the apparent size. It is not
possible for the plugin to work out the size itself since it doesn't see the whole
stream until it has all been streamed.
FILES
$plugindir/nbdkit-streaming-plugin.so
The plugin.
Use "nbdkit --dump-config" to find the location of $plugindir.
VERSION
"nbdkit-streaming-plugin" first appeared in nbdkit 1.2.
SEE ALSO
nbdkit(1), nbdkit-file-plugin(1), nbdkit-nbd-plugin(1), nbdkit-plugin(3),
nbdkit-captive(1), nbdcopy(1), nbdfuse(1), qemu-img(1).
AUTHORS
Richard W.M. Jones
COPYRIGHT
Copyright (C) 2014-2020 Red Hat Inc.
LICENSE
Redistribution and use in source and binary forms, with or without modification, are
permitted provided that the following conditions are met:
• Redistributions of source code must retain the above copyright notice, this list of
conditions and the following disclaimer.
• Redistributions in binary form must reproduce the above copyright notice, this list of
conditions and the following disclaimer in the documentation and/or other materials
provided with the distribution.
• Neither the name of Red Hat nor the names of its contributors may be used to endorse
or promote products derived from this software without specific prior written
permission.
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