1nbdkit-retry-filter(1) NBDKIT nbdkit-retry-filter(1)
2
6 nbdkit-retry-filter - reopen connection on error
7
9 nbdkit --filter=retry PLUGIN [retries=N] [retry-delay=N]
10 [retry-exponential=yes|no]
11 [retry-readonly=yes|no]
12
14 "nbdkit-retry-filter" is a filter that transparently reopens the plugin
15 connection when an error is encountered. It can be used to make long-
16 running copy operations reliable in the presence of temporary network
17 failures, without requiring any changes to the plugin or the NBD
18 client.
19 Several optional parameters are available to control:
21 · how many times we retry,
23 · the delay between retries, and whether we wait longer each time
25 (known as “exponential back-off”),
26 · if we reopen the plugin in read-only mode after the first failure.
28 The default (with no parameters) is designed to offer a happy medium
30 between recovering from short temporary failures but not doing anything
31 too bad when permanent or unrecoverable failures happen. The default
32 behaviour is: we retry 5 times with exponential back-off, waiting in
33 total about 1 minute before we give up.
34
36 In this example we copy and convert a large file using
37 nbdkit-ssh-plugin(1), qemu-img(1) and nbdkit-captive(1).
38 nbdkit -U - \
40 ssh host=remote.example.com /var/tmp/test.iso \
41 --filter=retry \
42 --run 'qemu-img convert -p -f raw $nbd -O qcow2 test.qcow2'
43 Without --filter=retry a temporary failure would cause the copy to fail
45 (for example, the remote host’s firewall is restarted causing the SSH
46 connection to be dropped). Adding this filter means that it may be
47 possible to transparently recover.
48
50 retries=N
51 The number of times any single operation will be retried before we
52 give up and fail the operation. The default is 5.
53 retry-delay=N
55 The number of seconds to wait before retrying. The default is 2
56 seconds.
57 retry-exponential=yes
59 Use exponential back-off. The retry delay is doubled between each
60 retry. This is the default.
61 retry-exponential=no
63 Do not use exponential back-off. The retry delay is the same
64 between each retry.
65 retry-readonly=yes
67 As soon as a failure occurs, switch the underlying plugin to read-
68 only mode for the rest of this connection. (A new NBD client
69 connection will still open the plugin in the original mode.)
70 retry-readonly=no
72 Do not change the read-write/read-only mode of the plugin when
73 retrying. This is the default.
74
76 $filterdir/nbdkit-retry-filter.so
77 The filter.
78 Use "nbdkit --dump-config" to find the location of $filterdir.
80
82 "nbdkit-retry-filter" first appeared in nbdkit 1.16.
83
85 nbdkit(1), nbdkit-filter(3), nbdkit-readahead-filter(1).
86
88 Richard W.M. Jones
89
91 Copyright (C) 2019 Red Hat Inc.
92
94 Redistribution and use in source and binary forms, with or without
95 modification, are permitted provided that the following conditions are
96 met:
97 · Redistributions of source code must retain the above copyright
99 notice, this list of conditions and the following disclaimer.
100 · Redistributions in binary form must reproduce the above copyright
102 notice, this list of conditions and the following disclaimer in the
103 documentation and/or other materials provided with the
104 distribution.
105 · Neither the name of Red Hat nor the names of its contributors may
107 be used to endorse or promote products derived from this software
108 without specific prior written permission.
109 THIS SOFTWARE IS PROVIDED BY RED HAT AND CONTRIBUTORS ''AS IS'' AND ANY
111 EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
112 IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
113 PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL RED HAT OR CONTRIBUTORS BE
114 LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
115 CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
116 SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR
117 BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY,
118 WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR
119 OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF
120 ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
121nbdkit-1.16.1 2019-12-03 nbdkit-retry-filter(1)