nbdkit-streaming-plugin - Man Page

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

License

Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met:

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.

Referenced By

nbdkit(1), nbdkit-plugin(3), nbdkit-release-notes-1.22(1).

2020-11-19 nbdkit-1.23.9