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NAME

nbdkit-streaming-plugin - nbdkit streaming plugin

SYNOPSIS

 nbdkit streaming write=PIPE [size=SIZE]
 nbdkit streaming read=PIPE [size=SIZE]

DEPRECATED

The streaming plugin is deprecated in nbdkit ≥ 1.26 and will be removed in nbdkit 1.30.

A suggested replacement is the libnbd program nbdcopy(1) which is able to stream to and from pipes, sockets and stdio. For example to stream from a qcow2 file to another program over a pipe, use:

 nbdcopy -- [ qemu-nbd -f qcow2 file.qcow2 ] - | hexdump -C

There are further examples in the nbdcopy(1) manual.

"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.

AUTHORS

Richard W.M. Jones

COPYRIGHT

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.

THIS SOFTWARE IS PROVIDED BY RED HAT AND CONTRIBUTORS ’’ AS IS ’’ AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL RED HAT OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES ( INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES ; LOSS OF USE, DATA, OR PROFITS ; OR BUSINESS INTERRUPTION ) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT ( INCLUDING NEGLIGENCE OR OTHERWISE ) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

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