Ubuntu Manpage: nbdkit-file-plugin

Provided by: nbdkit_1.36.3-1ubuntu10_amd64

NAME

       nbdkit-file-plugin - nbdkit file plugin

SYNOPSIS

        nbdkit file [file=]FILENAME
                    [cache=default|none] [fadvise=normal|random|sequential]

        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", chosen by the client using the NBD export name.

       If you use the "fd" or "dirfd" parameter then you can pass the file descriptor of a single disk or a
       directory to the plugin, inherited from the parent process.  This can be useful where special permissions
       or capabilities are needed to open the file descriptor, or you want to run nbdkit in a sandboxed
       environment.

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 descriptor.

       cache=default
       cache=none
           (nbdkit ≥ 1.22, not Windows)

           Using  "cache=none" tries to prevent the kernel from keeping parts of the file that have already been
           read or written in the page cache.

       dir=DIRECTORY
           (nbdkit ≥ 1.22, not Windows)

           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,
           where  the client can request a list of available exports using NBD_OPT_LIST.  A client that requests
           the default export ("") will be rejected.  However, you can use nbdkit-exportname-filter(1) to adjust
           what export names the client sees or uses as a default.  For security,  when  using  directory  mode,
           this plugin will not accept export names containing slash ("/").

       dirfd=FILE_DESCRIPTOR
           (nbdkit ≥ 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.

       fadvise=normal
       fadvise=random
       fadvise=sequential
           (nbdkit ≥ 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 ≥ 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=" is a magic config key and 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/naming-a-file#win32-device-namespaces.

NOTES

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.

Reducing evictions from the page cache
If the file is very large and you know the client will only read/write the file sequentially one time (eg
for making a single copy or backup) then this will stop other processes from being evicted from the page
cache:

nbdkit file disk.img fadvise=sequential cache=none

Only use fadvise=sequential if reading, and the reads are mainly sequential.

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_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_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 plugin 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.

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 modification, 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 notice, this list of conditions and
           the following disclaimer in the documentation 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 EXPRESS  OR  IMPLIED  WARRANTIES,
       INCLUDING,  BUT  NOT  LIMITED  TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
       PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL RED HAT OR CONTRIBUTORS BE LIABLE  FOR  ANY  DIRECT,  INDIRECT,
       INCIDENTAL,  SPECIAL,  EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
       SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
       ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT  LIABILITY,  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
       DAMAGE.

nbdkit-1.36.3                                      2024-03-31                              nbdkit-file-plugin(1)