1nbdkit-streaming-plugin(1) NBDKIT nbdkit-streaming-plugin(1)
2
6 nbdkit-streaming-plugin - nbdkit streaming plugin
7
9 nbdkit streaming write=PIPE [size=SIZE]
10 nbdkit streaming read=PIPE [size=SIZE]
12
14 "nbdkit-streaming-plugin" is a plugin for nbdkit(1) that can stream in
15 or out of a local pipe or socket. An alternate tool which can do
16 something similar to this is nbdcopy(1). To turn an NBD export into a
17 local file use nbdfuse(1).
18 If the NBD client opens the NBD port and writes from the start to the
20 end of the disk without seeking backwards, then you can turn that into
21 a stream of data on a local pipe or socket using:
22 nbdkit streaming write=./pipe
24 This can be visualised as:
26 ┌───────────┐
28 NBD │ nbdkit │ plugin streams
29 client ──────▶│ streaming │──────▶ data to ./pipe
30 writes │ plugin │
31 └───────────┘
32 If the NBD client opens the NBD port and reads from the start to the
34 end of the disk without seeking backwards, then you can turn a local
35 pipe or socket into a stream of data for that client:
36 nbdkit streaming read=./pipe
38 This can be visualised as:
40 ┌───────────┐
42 plugin streams │ nbdkit │ NBD
43 data from ──────▶│ streaming │──────▶ client
44 ./pipe │ plugin │ reads
45 └───────────┘
46 Note that ./pipe (or the local socket) sees raw data, it is not using
48 the NBD protocol. If you want to forward NBD to a local socket
49 connected to another NBD server, use nbdkit-nbd-plugin(1).
50 Combining this plugin with qemu tools
52 Typical usage is with qemu tools. The following command does not work
53 because the output is a pipe or socket:
54 $ mkfifo pipe
56 $ qemu-img convert -n input -O raw ./pipe
57 qemu-img: Could not open './pipe': A regular file was expected by
58 the 'file' driver, but something else was given
59 However this will work:
61 nbdkit -U - streaming write=./pipe \
63 --run ' qemu-img convert -n input -O raw $nbd '
64 ┌───────────┐ ┌───────────┐
66 reads │ qemu-img │ │ nbdkit │ plugin streams
67 input ──────▶│ convert │──────▶│ streaming │──────▶ data to ./pipe
68 file │ command │ │ plugin │
69 └───────────┘ ↑└───────────┘
70 Unix domain socket (-U -)
71 This only works because the output format is raw and qemu-img can write
73 that linearly (without seeking backwards or writing blocks out of
74 order). This approach cannot work for other formats such as qcow2
75 since those contain metadata that must be updated by seeking back to
76 the start of the file which is not possible if the output is a pipe.
77 The reverse is to get qemu-img to read from a pipe:
79 nbdkit -U - streaming read=./pipe \
81 --run ' qemu-img convert -f raw $nbd -O qcow2 output.qcow2 '
82 For use of the --run and -U - options, see nbdkit-captive(1).
84
86 Either "read" or "write" is required, but not both.
87 read=FILENAME
89 (nbdkit ≥ 1.22)
90 Read data stream from the named pipe or socket. If the pipe or
92 socket does not exist, then it is created (as a named FIFO),
93 otherwise the existing pipe or socket is opened and used.
94 write=FILENAME
96 Write data stream to the named pipe or socket. If the pipe or
97 socket does not exist, then it is created (as a named FIFO),
98 otherwise the existing pipe or socket is opened and used.
99 pipe=FILENAME
101 For backwards compatibility with nbdkit ≤ 1.20 this is a synonym
102 for "write=FILENAME".
103 size=SIZE
105 Specify the virtual size of the stream.
106 This parameter is optional. If not specified, then the virtual
108 disk appears to the client to be very large (effectively infinite).
109 Whether you need to specify this parameter depends on the client.
110 Some clients don't check the size and just write/stream, others do
111 checks or calculations based on the apparent size. It is not
112 possible for the plugin to work out the size itself since it
113 doesn't see the whole stream until it has all been streamed.
114
116 $plugindir/nbdkit-streaming-plugin.so
117 The plugin.
118 Use "nbdkit --dump-config" to find the location of $plugindir.
120
122 "nbdkit-streaming-plugin" first appeared in nbdkit 1.2.
123
125 nbdkit(1), nbdkit-file-plugin(1), nbdkit-nbd-plugin(1),
126 nbdkit-plugin(3), nbdkit-captive(1), nbdcopy(1), nbdfuse(1),
127 qemu-img(1).
128
130 Richard W.M. Jones
131
133 Copyright (C) 2014-2020 Red Hat Inc.
134
136 Redistribution and use in source and binary forms, with or without
137 modification, are permitted provided that the following conditions are
138 met:
139 · Redistributions of source code must retain the above copyright
141 notice, this list of conditions and the following disclaimer.
142 · Redistributions in binary form must reproduce the above copyright
144 notice, this list of conditions and the following disclaimer in the
145 documentation and/or other materials provided with the
146 distribution.
147 · Neither the name of Red Hat nor the names of its contributors may
149 be used to endorse or promote products derived from this software
150 without specific prior written permission.
151 THIS SOFTWARE IS PROVIDED BY RED HAT AND CONTRIBUTORS ''AS IS'' AND ANY
153 EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
154 IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
155 PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL RED HAT OR CONTRIBUTORS BE
156 LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
157 CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
158 SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR
159 BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY,
160 WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR
161 OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF
162 ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
163nbdkit-1.24.2 2021-03-02 nbdkit-streaming-plugin(1)