1nbdkit-truncate-filter(1)           NBDKIT           nbdkit-truncate-filter(1)
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NAME

6       nbdkit-truncate-filter - change the size of plugins
7

SYNOPSIS

9        nbdkit --filter=truncate plugin [truncate=SIZE]
10                                        [round-up=N] [round-down=N]
11

DESCRIPTION

13       "nbdkit-truncate-filter" is a filter that changes the size of the
14       underlying plugin.  It can:
15
16       ·   Make the plugin smaller (truncate it).  Use the "truncate=SIZE"
17           parameter to set the smaller size.
18
19       ·   Make the plugin larger (the additional bytes read back as zeroes).
20           Use "truncate=SIZE" to set the larger size.
21
22       ·   Round the size of the plugin up or down to the next multiple of
23           "N".  Use either "round-up=N" or "round-down=N".
24
25       A common use for this filter is to handle NBD clients which have a
26       problem dealing with device sizes which are not a multiple of 512
27       bytes.  Use "round-up=512" to round the size up to the next multiple of
28       512 bytes.  If the size is already a multiple of 512 bytes then this
29       has no effect.
30

PARAMETERS

32       truncate=SIZE
33           Set the absolute size in bytes of the apparent device.  This may be
34           smaller or larger or the same as the underlying plugin.
35
36           If the size is larger than the underlying plugin, reading the extra
37           space returns zeroes.  Writes are also permitted to the extra
38           space, but you must only write zeroes (any attempts to write non-
39           zero bytes will return an error back to the client).
40
41           This parameter is optional.
42
43       round-up=N
44           Round the size up to the next multiple of "N" bytes.  If the size
45           of the underlying plugin is already a multiple of "N" bytes, this
46           has no effect.
47
48           This option is useful when combined with other filters (such as
49           nbdkit-blocksize-filter(1), nbdkit-cache-filter(1), or
50           nbdkit-cow-filter(1)) that normally round down due to a larger
51           granularity, in order to access the last few bytes of a file that
52           would otherwise be rendered inaccessible.
53
54           This parameter is optional.
55
56       round-down=N
57           Round the size down to a multiple of "N" bytes.  If the size of the
58           underlying plugin is already a multiple of "N" bytes, this has no
59           effect.
60
61           This parameter is optional.
62

EXAMPLES

64       Serve a file as if it were a block device that insists on 4k alignment,
65       while still allowing access to any unaligned bytes at the end of the
66       file:
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68        nbdkit --filter=blocksize --filter=truncate file /path/to/file \
69        minblock=4k round-up=4k
70

FILES

72       $filterdir/nbdkit-truncate-filter.so
73           The filter.
74
75           Use "nbdkit --dump-config" to find the location of $filterdir.
76

VERSION

78       "nbdkit-truncate-filter" first appeared in nbdkit 1.6.
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SEE ALSO

81       nbdkit(1), nbdkit-file-plugin(1), nbdkit-filter(3),
82       nbdkit-blocksize-filter(1), nbdkit-cache-filter(1),
83       nbdkit-cow-filter(1), nbdkit-offset-filter(1),
84       nbdkit-partition-filter(1).
85

AUTHORS

87       Richard W.M. Jones
88

90       Copyright (C) 2018 Red Hat Inc.
91

LICENSE

93       Redistribution and use in source and binary forms, with or without
94       modification, are permitted provided that the following conditions are
95       met:
96
97       ·   Redistributions of source code must retain the above copyright
98           notice, this list of conditions and the following disclaimer.
99
100       ·   Redistributions in binary form must reproduce the above copyright
101           notice, this list of conditions and the following disclaimer in the
102           documentation and/or other materials provided with the
103           distribution.
104
105       ·   Neither the name of Red Hat nor the names of its contributors may
106           be used to endorse or promote products derived from this software
107           without specific prior written permission.
108
109       THIS SOFTWARE IS PROVIDED BY RED HAT AND CONTRIBUTORS ''AS IS'' AND ANY
110       EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
111       IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
112       PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL RED HAT OR CONTRIBUTORS BE
113       LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
114       CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
115       SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR
116       BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY,
117       WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR
118       OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF
119       ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
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123nbdkit-1.24.2                     2021-03-02         nbdkit-truncate-filter(1)
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