nbdkit-streaming-plugin1

Usage

Overview

"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).

 $ 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

 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 -)

 nbdkit -U - streaming read=./pipe \
        --run ' qemu-img convert -f raw $nbd -O qcow2 output.qcow2 '

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.