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 An alternative and more fine-grained filter is
21 nbdkit-retry-request-filter(1) which will retry only single requests
22 that fail.
23 Several optional parameters are available to control:
25 • how many times we retry,
27 • the delay between retries, and whether we wait longer each time
29 (known as “exponential back-off”),
30 • if we reopen the plugin in read-only mode after the first failure.
32 The default (with no parameters) is designed to offer a happy medium
34 between recovering from short temporary failures but not doing anything
35 too bad when permanent or unrecoverable failures happen. The default
36 behaviour is: we retry 5 times with exponential back-off, waiting in
37 total about 1 minute before we give up.
38
40 In this example we copy and convert a large file using
41 nbdkit-ssh-plugin(1), qemu-img(1) and nbdkit-captive(1).
42 nbdkit -U - \
44 ssh host=remote.example.com /var/tmp/test.iso \
45 --filter=retry \
46 --run 'qemu-img convert -p -f raw $nbd -O qcow2 test.qcow2'
47 Without --filter=retry a temporary failure would cause the copy to fail
49 (for example, the remote host’s firewall is restarted causing the SSH
50 connection to be dropped). Adding this filter means that it may be
51 possible to transparently recover.
52
54 retries=N
55 The number of times any single operation will be retried before we
56 give up and fail the operation. The default is 5.
57 retry-delay=N
59 The number of seconds to wait before retrying. The default is 2
60 seconds.
61 retry-exponential=yes
63 Use exponential back-off. The retry delay is doubled between each
64 retry. This is the default.
65 retry-exponential=no
67 Do not use exponential back-off. The retry delay is the same
68 between each retry.
69 retry-readonly=yes
71 As soon as a failure occurs, switch the underlying plugin to read-
72 only mode for the rest of this connection. (A new NBD client
73 connection will still open the plugin in the original mode.)
74 retry-readonly=no
76 Do not change the read-write/read-only mode of the plugin when
77 retrying. This is the default.
78
80 $filterdir/nbdkit-retry-filter.so
81 The filter.
82 Use "nbdkit --dump-config" to find the location of $filterdir.
84
86 "nbdkit-retry-filter" first appeared in nbdkit 1.16.
87
89 nbdkit(1), nbdkit-filter(3), nbdkit-readahead-filter(1),
90 nbdkit-retry-request-filter(1).
91
93 Richard W.M. Jones
94
96 Copyright (C) 2019 Red Hat Inc.
97
99 Redistribution and use in source and binary forms, with or without
100 modification, are permitted provided that the following conditions are
101 met:
102 • Redistributions of source code must retain the above copyright
104 notice, this list of conditions and the following disclaimer.
105 • Redistributions in binary form must reproduce the above copyright
107 notice, this list of conditions and the following disclaimer in the
108 documentation and/or other materials provided with the
109 distribution.
110 • Neither the name of Red Hat nor the names of its contributors may
112 be used to endorse or promote products derived from this software
113 without specific prior written permission.
114 THIS SOFTWARE IS PROVIDED BY RED HAT AND CONTRIBUTORS ''AS IS'' AND ANY
116 EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
117 IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
118 PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL RED HAT OR CONTRIBUTORS BE
119 LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
120 CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
121 SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR
122 BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY,
123 WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR
124 OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF
125 ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
126nbdkit-1.32.5 2023-01-03 nbdkit-retry-filter(1)