1nbdkit-file-plugin(1) NBDKIT nbdkit-file-plugin(1)
2
6 nbdkit-file-plugin - nbdkit file plugin
7
9 nbdkit file [file=]FILENAME
10 [cache=default|none] [fadvise=normal|random|sequential]
11 nbdkit file dir=DIRECTORY
13 nbdkit file fd=FILE_DESCRIPTOR
15 nbdkit file dirfd=FILE_DESCRIPTOR
17
19 "nbdkit-file-plugin" is a file serving plugin for nbdkit(1).
20 It serves the named "FILENAME" over NBD. Local block devices (eg.
22 /dev/sda) may also be served.
23 If you use the "dir" parameter the plugin works in a different mode
25 where it serves files from the given "DIRECTORY", chosen by the client
26 using the NBD export name.
27 If you use the "fd" or "dirfd" parameter then you can pass the file
29 descriptor of a single disk or a directory to the plugin, inherited
30 from the parent process. This can be useful where special permissions
31 or capabilities are needed to open the file descriptor, or you want to
32 run nbdkit in a sandboxed environment.
33
35 Exactly one of file, dir, fd or dirfd must be given. This controls the
36 mode of the plugin, either serving a single file, the files in a
37 directory, a single file descriptor, or the files in the directory of
38 the file descriptor.
39 cache=default
41 cache=none
42 (nbdkit ≥ 1.22, not Windows)
43 Using "cache=none" tries to prevent the kernel from keeping parts
45 of the file that have already been read or written in the page
46 cache.
47 dir=DIRECTORY
49 (nbdkit ≥ 1.22, not Windows)
50 Serve all regular files and block devices located directly inside
52 the directory named "DIRECTORY", including those found by following
53 symbolic links. Other special files in the directory (such as
54 subdirectories, pipes, or Unix sockets) are ignored.
55 When this mode is used, the file to be served is chosen by the
57 export name passed by the client, where the client can request a
58 list of available exports using NBD_OPT_LIST. A client that
59 requests the default export ("") will be rejected. However, you
60 can use nbdkit-exportname-filter(1) to adjust what export names the
61 client sees or uses as a default. For security, when using
62 directory mode, this plugin will not accept export names containing
63 slash ("/").
64 dirfd=FILE_DESCRIPTOR
66 (nbdkit ≥ 1.34, not Windows)
67 This is like the dir option, but instead of specifying the
69 directory by name, the parent process should open the directory and
70 pass this file descriptor by inheritance to nbdkit.
71 fadvise=normal
73 fadvise=random
74 fadvise=sequential
75 (nbdkit ≥ 1.22, not Windows)
76 This optional flag hints to the kernel that you will access the
78 file normally, or in a random order, or sequentially. The exact
79 behaviour depends on your operating system, but for Linux using
80 "normal" causes the kernel to read-ahead, "sequential" causes the
81 kernel to read-ahead twice as much as "normal", and "random" turns
82 off read-ahead. See also posix_fadvise(2).
83 The default is "normal".
85 fd=FILE_DESCRIPTOR
87 (nbdkit ≥ 1.34, not Windows)
88 The parameter is the number of a file descriptor. Serve the file
90 or device already open on this file descriptor. The file
91 descriptor is usually inherited from the parent process.
92 [file=]FILENAME
94 Serve the file named "FILENAME". A local block device name can
95 also be used here. When this mode is used, the export name
96 requested by the client is ignored.
97 "file=" is a magic config key and may be omitted in most cases.
99 See "Magic parameters" in nbdkit(1).
100 [file=]\\.\C:
102 [file=]\\.\Volume
103 [file=]\\.\PhysicalDiskN
104 [file=]\\.\CdRomN
105 (Windows only)
106 Serve the Windows volume specified by the device name. See:
108 https://docs.microsoft.com/en-us/windows/win32/fileio/naming-a-file#win32-device-namespaces.
109
111 Optimizing for random or sequential access
112 If you know in advance that the NBD client will access the file
113 randomly or only sequentially then you can hint that to the kernel
114 using:
115 nbdkit file disk.img fadvise=random
117 nbdkit file disk.img fadvise=sequential
118 As described in the "PARAMETERS" section above, on Linux this disables
120 or increases the amount of read-ahead that the kernel does.
121 Reducing evictions from the page cache
123 If the file is very large and you know the client will only read/write
124 the file sequentially one time (eg for making a single copy or backup)
125 then this will stop other processes from being evicted from the page
126 cache:
127 nbdkit file disk.img fadvise=sequential cache=none
129 Only use fadvise=sequential if reading, and the reads are mainly
131 sequential.
132 Files on tmpfs
134 If you want to expose a file that resides on a file system known to
135 have poor lseek(2) performance when searching for holes ("tmpfs" is
136 known to be one such file system), you can use
137 nbdkit-noextents-filter(1) to avoid the penalty of probing for holes.
138 Plugin --dump-plugin output
140 You can obtain extra information about how the file plugin was compiled
141 by doing:
142 nbdkit file --dump-plugin
144 Some of the fields which may appear are listed below. Note these are
146 for information only and may be changed or removed at any time in the
147 future.
148 "file_blksszget=yes"
150 "file_blkzeroout=yes"
151 If both set, the plugin may be able to efficiently zero ranges of
152 block devices, where the driver and block device itself supports
153 this.
154 "file_extents=yes"
156 If set, the plugin can read file extents.
157 "file_falloc_fl_punch_hole=yes"
159 If set, the plugin may be able to punch holes (make sparse) files
160 and block devices.
161 "file_falloc_fl_zero_range=yes"
163 If set, the plugin may be able to efficiently zero ranges of files
164 and block devices.
165 "winfile=yes"
167 If present, this is the Windows version of the file plugin with
168 reduced functionality and some special Windows-only features, as
169 noted in this manual.
170 Windows sparse files
172 This plugin supports sparse files on Windows (with hole punching).
173 However for this to work the files must already have the sparse
174 property, the plugin will not make existing files sparse. Use the
175 "fsutil sparse" command to control the sparseness property of files.
176 Old "rdelay" and "wdelay" parameters.
178 Before nbdkit supported filters (< 1.2) this plugin had extra
179 parameters "rdelay" and "wdelay" to insert delays. These parameters
180 have been moved to nbdkit-delay-filter(1). Modify the command line to
181 add --filter=delay in order to use these parameters.
182 Concatenating files
184 To concatenate and export multiple files, use nbdkit-split-plugin(1).
185
187 -D file.zero=1
188 This enables very verbose debugging of the NBD zero request. This
189 can be used to tell if the file plugin is able to zero ranges in
190 the file or block device efficiently or not.
191
193 $plugindir/nbdkit-file-plugin.so
194 The plugin.
195 Use "nbdkit --dump-config" to find the location of $plugindir.
197
199 "nbdkit-file-plugin" first appeared in nbdkit 1.0.
200
202 nbdkit(1), nbdkit-plugin(3), nbdkit-split-plugin(1),
203 nbdkit-partitioning-plugin(1), nbdkit-tmpdisk-plugin(1),
204 nbdkit-exportname-filter(1), nbdkit-fua-filter(1),
205 nbdkit-luks-filter(1), nbdkit-noextents-filter(1).
206
208 Eric Blake
209 Nir Soffer
211 Richard W.M. Jones
213
215 Copyright Red Hat
216
218 Redistribution and use in source and binary forms, with or without
219 modification, are permitted provided that the following conditions are
220 met:
221 • Redistributions of source code must retain the above copyright
223 notice, this list of conditions and the following disclaimer.
224 • Redistributions in binary form must reproduce the above copyright
226 notice, this list of conditions and the following disclaimer in the
227 documentation and/or other materials provided with the
228 distribution.
229 • Neither the name of Red Hat nor the names of its contributors may
231 be used to endorse or promote products derived from this software
232 without specific prior written permission.
233 THIS SOFTWARE IS PROVIDED BY RED HAT AND CONTRIBUTORS ''AS IS'' AND ANY
235 EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
236 IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
237 PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL RED HAT OR CONTRIBUTORS BE
238 LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
239 CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
240 SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR
241 BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY,
242 WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR
243 OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF
244 ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
245nbdkit-1.36.2 2023-11-26 nbdkit-file-plugin(1)