nbdkit-file-plugin(1) NBDKIT nbdkit-file-plugin(1) NAME nbdkit-file-plugin - nbdkit file plugin SYNOPSIS nbdkit file [file=]FILENAME [fadvise=normal|random|sequential] [reduce-memory-pressure=true] nbdkit file dir=DIRECTORY nbdkit file fd=FILE_DESCRIPTOR nbdkit file dirfd=FILE_DESCRIPTOR DESCRIPTION "nbdkit-file-plugin" is a file serving plugin for nbdkit(1). It serves the named "FILENAME" over NBD. Local block devices (eg. /dev/sda) may also be served. If you use the "dir" parameter the plugin works in a different mode where it serves files from the given "DIRECTORY". The filename is chosen by the client sending an NBD export name. If you use the "fd" or "dirfd" parameter then you can pass the file de- scriptor of a single disk or a directory to the plugin, inherited from the parent process. This can be useful where special permissions or capabili- ties are needed to open the file descriptor, or you want to run nbdkit in a sandboxed environment. The file is writable unless either the -r command line option, or nbd- kit-readonly-filter(1) is used. PARAMETERS Exactly one of file, dir, fd or dirfd must be given. This controls the mode of the plugin, either serving a single file, the files in a directory, a single file descriptor, or the files in the directory of the file de- scriptor. dir=DIRECTORY (nbdkit aY 1.22, not Windows) Serve all regular files and block devices located directly inside the directory named "DIRECTORY", including those found by following sym- bolic links. Other special files in the directory (such as subdirecto- ries, pipes, or Unix sockets) are ignored. See "Serving multiple files and block devices" below. dirfd=FILE_DESCRIPTOR (nbdkit aY 1.34, not Windows) This is like the "dir=" option, but instead of specifying the directory by name, the parent process should open the directory and pass this file descriptor by inheritance to nbdkit. See "Serving multiple files and block devices" below. fadvise=normal fadvise=random fadvise=sequential (nbdkit aY 1.22, not Windows) This optional flag hints to the kernel that you will access the file normally, or in a random order, or sequentially. The exact behaviour depends on your operating system, but for Linux using "normal" causes the kernel to read-ahead, "sequential" causes the kernel to read-ahead twice as much as "normal", and "random" turns off read-ahead. See also posix_fadvise(2). The default is "normal". fd=FILE_DESCRIPTOR (nbdkit aY 1.34, not Windows) The parameter is the number of a file descriptor. Serve the file or device already open on this file descriptor. The file descriptor is usually inherited from the parent process. [file=]FILENAME Serve the file named "FILENAME". A local block device name can also be used here. When this mode is used, the export name requested by the client is ignored. "file=" prefix may be omitted in most cases. See "Magic parameters" in nbdkit(1). [file=]\\.\C: [file=]\\.\Volume [file=]\\.\PhysicalDiskN [file=]\\.\CdRomN (Windows only) Serve the Windows volume specified by the device name. See: https://docs.microsoft.com/en-us/windows/win32/fileio/nam- ing-a-file#win32-device-namespaces. reduce-memory-pressure=true (nbdkit aY 1.22, not Windows) If the file is very large and the plugin will read or write the whole file, this can cause the file to be cached in memory (in the page cache on Linux), causing other files and processes to be evicted, leading to overall slow system performance and I/O storms. If you know that the file will only be read/written once and will not be needed afterwards (so keeping it in memory afterwards is pointless) use "reduce-memory-pressure=true". This causes reads and writes to minimize use of the page cache. The default is false, which uses the kernel's normal page cache strat- egy. See also "fadvise" above which can be used to hint the access pattern. In nbdkit 1.22 - 1.44 this option was called "cache=none", but that caused a lot of confusion for people used to qemu and kernel caching modes, which are not related to this setting. NOTES Serving multiple files and block devices Using "dir=DIRECTORY" (or "dirfd=DIRFD") you can serve all regular files and block devices located directly inside the directory named "DIRECTORY", including those found by following symbolic links. Other special files in the directory (such as subdirectories, pipes, or Unix sockets) are ignored. When this mode is used, the file to be served is chosen by the export name passed by the client. For security, when using directory mode, this plugin will not accept export names containing slash ("/"). For example: $ ls -l /var/tmp/exports total 0 -rw-r--r--. 1 rjones rjones 1048576 Dec 14 15:34 disk1 -rw-r--r--. 1 rjones rjones 2097152 Dec 14 15:34 disk2 lrwxrwxrwx. 1 rjones rjones 9 Dec 14 15:35 sda1 -> /dev/sda1 $ nbdkit file dir=/var/tmp/exports will serve three exports called "disk1", "disk2" and "sda1". The first two are regular files and the last is a block device. The NBD URIs of the three exports are: * "nbd://localhost/disk1" * "nbd://localhost/disk2" * "nbd://localhost/sda1" You can add or remove files or symbolic links from the directory while nbd- kit is running. To list exports, use nbdinfo(1) --list option, for example: $ nbdinfo --list nbd://localhost protocol: newstyle-fixed without TLS, using structured packets export="disk1": export-size: 1048576 (1M) uri: nbd://localhost:10809/disk1 [etc] An NBD client can request a list of available exports using "NBD_OPT_LIST". For libnbd clients see nbd_opt_list(3). A client that requests the default export ("") will be rejected. However, you can use nbdkit-exportname-filter(1) to adjust the default export as well as other transformations of export names. For example to make /var/tmp/exports/disk1 be the default export: nbdkit file dir=/var/tmp/exports \ --filter=exportname default-export=disk1 Optimizing for random or sequential access If you know in advance that the NBD client will access the file randomly or only sequentially then you can hint that to the kernel using: nbdkit file disk.img fadvise=random nbdkit file disk.img fadvise=sequential As described in the "PARAMETERS" section above, on Linux this disables or increases the amount of read-ahead that the kernel does. Files on tmpfs If you want to expose a file that resides on a file system known to have poor lseek(2) performance when searching for holes ("tmpfs" is known to be one such file system), you can use nbdkit-noextents-filter(1) to avoid the penalty of probing for holes. Plugin --dump-plugin output You can obtain extra information about how the file plugin was compiled by doing: nbdkit file --dump-plugin Some of the fields which may appear are listed below. Note these are for information only and may be changed or removed at any time in the future. "file_block_size=yes" If set, the plugin has support for getting the minimum and preferred I/O size of block devices. "file_blkrotational=yes" If set, the plugin has support for getting the rotational property of block devices. "file_blksszget=yes" "file_blkzeroout=yes" If both set, the plugin may be able to efficiently zero ranges of block devices, where the driver and block device itself supports this. "file_blkdiscard=yes" If set, the plugin may be able to efficiently trim ranges of block de- vices, where the driver and block device itself supports this. "file_extents=yes" If set, the plugin can read file extents. "file_falloc_fl_punch_hole=yes" If set, the plugin may be able to punch holes (make sparse) files and block devices. "file_falloc_fl_zero_range=yes" If set, the plugin may be able to efficiently zero ranges of files and block devices. "winfile=yes" If present, this is the Windows version of the file plugin with reduced functionality and some special Windows-only features, as noted in this manual. Windows sparse files This plugin supports sparse files on Windows (with hole punching). However for this to work the files must already have the sparse property, the plu- gin will not make existing files sparse. Use the "fsutil sparse" command to control the sparseness property of files. Old "rdelay" and "wdelay" parameters. Before nbdkit supported filters (< 1.2) this plugin had extra parameters "rdelay" and "wdelay" to insert delays. These parameters have been moved to nbdkit-delay-filter(1). Modify the command line to add --filter=delay in order to use these parameters. Concatenating files To concatenate and export multiple files, use nbdkit-split-plugin(1). DEBUG FLAG -D file.zero=1 This enables very verbose debugging of the NBD zero request. This can be used to tell if the file plugin is able to zero ranges in the file or block device efficiently or not. FILES $plugindir/nbdkit-file-plugin.so The plugin. Use "nbdkit --dump-config" to find the location of $plugindir. VERSION "nbdkit-file-plugin" first appeared in nbdkit 1.0. SEE ALSO nbdkit(1), nbdkit-plugin(3), nbdkit-nfs-plugin(1), nbdkit-partitioning-plu- gin(1), nbdkit-split-plugin(1), nbdkit-tmpdisk-plugin(1), nbdkit-count-fil- ter(1), nbdkit-delay-filter(1), nbdkit-exportname-filter(1), nbd- kit-fua-filter(1), nbdkit-luks-filter(1), nbdkit-noextents-filter(1), nbd- kit-readonly-filter(1), nbdinfo(1). AUTHORS Eric Blake Nir Soffer Richard W.M. Jones COPYRIGHT Copyright Red Hat LICENSE Redistribution and use in source and binary forms, with or without modifi- cation, are permitted provided that the following conditions are met: * Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer. * Redistributions in binary form must reproduce the above copyright no- tice, this list of conditions and the following disclaimer in the docu- mentation and/or other materials provided with the distribution. * Neither the name of Red Hat nor the names of its contributors may be used to endorse or promote products derived from this software without specific prior written permission. THIS SOFTWARE IS PROVIDED BY RED HAT AND CONTRIBUTORS ''AS IS'' AND ANY EX- PRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DIS- CLAIMED. IN NO EVENT SHALL RED HAT OR CONTRIBUTORS BE LIABLE FOR ANY DI- RECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SER- VICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABIL- ITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAM- AGE. nbdkit-1.46.2 2026-08-30 nbdkit-file-plugin(1)
NAME | SYNOPSIS | DESCRIPTION | PARAMETERS | NOTES | DEBUG FLAG | FILES | VERSION | SEE ALSO | AUTHORS | COPYRIGHT | LICENSE
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