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.

SEE ALSO

       nbdkit(1), nbdkit-plugin(3), nbdkit-split-plugin(1), nbdkit-partitioning-plugin(1),
       nbdkit-tmpdisk-plugin(1), nbdkit-exportname-filter(1), nbdkit-fua-filter(1),
       nbdkit-luks-filter(1), nbdkit-noextents-filter(1).

AUTHORS

       Eric Blake

       Nir Soffer

       Richard W.M. Jones

COPYRIGHT

       Copyright Red Hat

LICENSE

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