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 This filter can be combined with nbdkit-blocksize-policy-filter(1) to
28 advertise different block sizes to the client.
29
31 The nbdkit-blocksize-filter accepts the following parameters.
32 minblock=SIZE
34 The minimum block size and alignment to pass to the plugin. This
35 must be a power of two, and no larger than 64k. If omitted, this
36 defaults to the minimum block size of the underlying plugin, or 1
37 if the plugin did not report a minimum block size. The filter
38 rounds up read requests to alignment boundaries, performs read-
39 modify-write cycles for any unaligned head or tail of a write or
40 zero request, and silently ignores any unaligned head or tail of a
41 trim request. The filter also truncates the plugin size down to an
42 aligned value (as it cannot safely operate on the unaligned tail).
43 If you need to round the image size up instead to access the last
44 few bytes, combine this filter with nbdkit-truncate-filter(1).
45 This parameter understands the suffix 'k' for 1024.
47 maxdata=SIZE
49 The maximum block size for any single transaction with data (read
50 and write). If omitted, this defaults to the minimum of 64
51 megabytes (that is, the nbdkit maximum) or any maximum reported by
52 the underlying plugin. This need not be a power of two, but must
53 be an integer multiple of "minblock". The filter fragments any
54 larger client request into multiple plugin requests.
55 This parameter understands the suffixes 'k', 'M', and 'G' for
57 powers of 1024.
58 maxlen=SIZE
60 The maximum length for any single transaction without data (trim,
61 zero, cache or extents). If omitted, this defaults to 0xffffffff
62 rounded down to "minblock" alignment (that is, the inherent 32-bit
63 limit of the NBD protocol). This need not be a power of two, but
64 must be an integer multiple of "minblock", and should be at least
65 as large as "maxdata". The filter fragments any larger client
66 request into multiple plugin requests.
67 This parameter understands the suffixes 'k', 'M', and 'G' for
69 powers of 1024.
70
72 Allow an arbitrary client to use the VDDK plugin (which is limited to
73 512-byte blocks), without having to fix the client to avoid sending
74 unaligned requests:
75 nbdkit --filter=blocksize vddk minblock=512 file=/absolute/path/to/file.vmdk
77 Allow an arbitrary client tuned to nbdkit's 64 megabyte sizing to
79 connect to a remote server that insists on 32 megabyte sizing, via the
80 nbd plugin:
81 nbdkit --filter=blocksize nbd maxdata=32M socket=/path/to/socket
83 Serve a file as if it were a block device that insists on 4k alignment,
85 while still allowing access to any unaligned bytes at the end of the
86 file:
87 nbdkit --filter=blocksize --filter=truncate file /path/to/file \
89 minblock=4k round-up=4k
90
92 $filterdir/nbdkit-blocksize-filter.so
93 The filter.
94 Use "nbdkit --dump-config" to find the location of $filterdir.
96
98 "nbdkit-blocksize-filter" first appeared in nbdkit 1.4.
99
101 nbdkit(1), nbdkit-nbd-plugin(1), nbdkit-vddk-plugin(1),
102 nbdkit-filter(3), nbdkit-blocksize-policy-filter(1),
103 nbdkit-truncate-filter(1).
104
106 Eric Blake
107
109 Copyright (C) 2018-2022 Red Hat Inc.
110
112 Redistribution and use in source and binary forms, with or without
113 modification, are permitted provided that the following conditions are
114 met:
115 • Redistributions of source code must retain the above copyright
117 notice, this list of conditions and the following disclaimer.
118 • Redistributions in binary form must reproduce the above copyright
120 notice, this list of conditions and the following disclaimer in the
121 documentation and/or other materials provided with the
122 distribution.
123 • Neither the name of Red Hat nor the names of its contributors may
125 be used to endorse or promote products derived from this software
126 without specific prior written permission.
127 THIS SOFTWARE IS PROVIDED BY RED HAT AND CONTRIBUTORS ''AS IS'' AND ANY
129 EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
130 IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
131 PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL RED HAT OR CONTRIBUTORS BE
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136 WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR
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139nbdkit-1.32.5 2023-01-03 nbdkit-blocksize-filter(1)