1nbdkit-blocksize-filter(1) NBDKIT nbdkit-blocksize-filter(1)
2
6 nbdkit-blocksize-filter - nbdkit blocksize filter
7
9 nbdkit --filter=blocksize plugin [minblock=SIZE] [maxdata=SIZE] \
10 [maxlen=SIZE] [plugin-args...]
11
13 "nbdkit-blocksize-filter" is a filter that ensures various block size
14 limits are met on transactions presented to the plugin. The NBD
15 protocol permits clients to send requests with a granularity as small
16 as 1 byte or as large as nearly 4 gigabytes, although it suggests that
17 portable clients should align requests to 512 bytes and not exceed 32
18 megabytes without prior coordination with the server.
19 Meanwhile, some plugins require requests to be aligned to 512-byte
21 multiples, or may enforce a maximum transaction size to bound the time
22 or memory resources spent by any one command (note that nbdkit itself
23 refuses a read or write larger than 64 megabytes, while many other NBD
24 servers limit things to 32 megabytes). The blocksize filter can be
25 used to modify the client requests to meet the plugin restrictions.
26
28 The nbdkit-blocksize-filter accepts the following parameters.
29 minblock=SIZE
31 The minimum block size and alignment to pass to the plugin. This
32 must be a power of two, and no larger than 64k. If omitted, this
33 defaults to 1 (that is, no minimum size restrictions). The filter
34 rounds up read requests to alignment boundaries, performs read-
35 modify-write cycles for any unaligned head or tail of a write or
36 zero request, and silently ignores any unaligned head or tail of a
37 trim request. The filter also truncates the plugin size down to an
38 aligned value (as it cannot safely operate on the unaligned tail).
39 If you need to round the image size up instead to access the last
40 few bytes, combine this filter with nbdkit-truncate-filter(1).
41 This parameter understands the suffix 'k' for 1024.
43 maxdata=SIZE
45 The maximum block size for any single transaction with data (read
46 and write). If omitted, this defaults to 64 megabytes (that is,
47 the nbdkit maximum). This need not be a power of two, but must be
48 an integer multiple of "minblock". The filter fragments any larger
49 client request into multiple plugin requests.
50 This parameter understands the suffixes 'k', 'M', and 'G' for
52 powers of 1024.
53 maxlen=SIZE
55 The maximum length for any single transaction without data (trim,
56 zero or extents). If omitted, this defaults to 0xffffffff rounded
57 down to "minsize" alignment (that is, the inherent 32-bit limit of
58 the NBD protocol). This need not be a power of two, but must be an
59 integer multiple of "minblock", and should be at least as large as
60 "maxdata". The filter fragments any larger client request into
61 multiple plugin requests.
62 This parameter understands the suffixes 'k', 'M', and 'G' for
64 powers of 1024.
65
67 Allow an arbitrary client to use the VDDK plugin (which is limited to
68 512-byte blocks), without having to fix the client to avoid sending
69 unaligned requests:
70 nbdkit --filter=blocksize vddk minblock=512 file=/absolute/path/to/file.vmdk
72 Allow an arbitrary client tuned to nbdkit's 64 megabyte sizing to
74 connect to a remote server that insists on 32 megabyte sizing, via the
75 nbd plugin:
76 nbdkit --filter=blocksize nbd maxdata=32M socket=/path/to/socket
78 Serve a file as if it were a block device that insists on 4k alignment,
80 while still allowing access to any unaligned bytes at the end of the
81 file:
82 nbdkit --filter=blocksize --filter=truncate file /path/to/file \
84 minblock=4k round-up=4k
85
87 $filterdir/nbdkit-blocksize-filter.so
88 The filter.
89 Use "nbdkit --dump-config" to find the location of $filterdir.
91
93 "nbdkit-blocksize-filter" first appeared in nbdkit 1.4.
94
96 nbdkit(1), nbdkit-nbd-plugin(1), nbdkit-vddk-plugin(1),
97 nbdkit-filter(3), nbdkit-truncate-filter(1).
98
100 Eric Blake
101
103 Copyright (C) 2018 Red Hat Inc.
104
106 Redistribution and use in source and binary forms, with or without
107 modification, are permitted provided that the following conditions are
108 met:
109 • Redistributions of source code must retain the above copyright
111 notice, this list of conditions and the following disclaimer.
112 • Redistributions in binary form must reproduce the above copyright
114 notice, this list of conditions and the following disclaimer in the
115 documentation and/or other materials provided with the
116 distribution.
117 • Neither the name of Red Hat nor the names of its contributors may
119 be used to endorse or promote products derived from this software
120 without specific prior written permission.
121 THIS SOFTWARE IS PROVIDED BY RED HAT AND CONTRIBUTORS ''AS IS'' AND ANY
123 EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
124 IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
125 PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL RED HAT OR CONTRIBUTORS BE
126 LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
127 CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
128 SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR
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130 WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR
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132 ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
133nbdkit-1.25.8 2021-05-25 nbdkit-blocksize-filter(1)