NAME

       nbdkit-file-plugin - nbdkit file plugin

SYNOPSIS

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

        nbdkit file dir=DIRNAME

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.  It may also be used to serve any file within a given "DIRECTORY", according to
       which export name the guest requests.

PARAMETERS

       One of file or dir must be given to determine which mode the server will use.

       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.

       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".

       [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.

       dir=DIRNAME
           (nbdkit ≥ 1.22, not Windows)

           Serve all regular files and block devices located directly within the directory named
           "DIRNAME", including those found by following symbolic links.  Other special files in
           the directory (such as subdirectories, fifos, 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 ("/").

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

   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_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 which lacks certain
           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-noextents-filter(1).

AUTHORS

       Eric Blake

       Nir Soffer

       Richard W.M. Jones

COPYRIGHT

       Copyright (C) 2013-2020 Red Hat Inc.

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
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