nbdkit file [file=]FILENAME [cache=default|none] [fadvise=normal|random|sequential] nbdkit file dir=DIRNAME
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.
One of file or dir must be given to determine which mode the server will use.
(nbdkit ≥ 1.22, not Windows)
cache=nonetries to prevent the kernel from keeping parts of the file that have already been read or written in the page cache.
(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
normalcauses the kernel to read-ahead,
sequentialcauses the kernel to read-ahead twice as much as
randomturns off read-ahead. See also posix_fadvise(2).
The default is
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).
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.
(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 (
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.
If both set, the plugin may be able to efficiently zero ranges of block devices, where the driver and block device itself supports this.
If set, the plugin may be able to punch holes (make sparse) files and block devices.
If set, the plugin may be able to efficiently zero ranges of files and block devices.
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
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.
To concatenate and export multiple files, use nbdkit-split-plugin(1).
- -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.
nbdkit --dump-configto find the location of
nbdkit-file-plugin first appeared in nbdkit 1.0.
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).
Richard W.M. Jones
Copyright (C) 2013-2020 Red Hat Inc.
Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met:
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- 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.
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nbdkit(1), nbdkit-cache-filter(1), nbdkit-cow-filter(1), nbdkit-curl-plugin(1), nbdkit-ddrescue-filter(1), nbdkit-delay-filter(1), nbdkit-error-filter(1), nbdkit-eval-plugin(1), nbdkit-exportname-filter(1), nbdkit-floppy-plugin(1), nbdkit-fua-filter(1), nbdkit-gzip-filter(1), nbdkit-iso-plugin(1), nbdkit-linuxdisk-plugin(1), nbdkit-log-filter(1), nbdkit-loop(1), nbdkit-memory-plugin(1), nbdkit-nocache-filter(1), nbdkit-noextents-filter(1), nbdkit-noparallel-filter(1), nbdkit-nozero-filter(1), nbdkit-offset-filter(1), nbdkit-ondemand-plugin(1), nbdkit-partition-filter(1), nbdkit-partitioning-plugin(1), nbdkit-plugin(3), nbdkit-release-notes-1.22(1), nbdkit-release-notes-1.6(1), nbdkit-split-plugin(1), nbdkit-streaming-plugin(1), nbdkit-swab-filter(1), nbdkit-tar-filter(1), nbdkit-tmpdisk-plugin(1), nbdkit-torrent-plugin(1), nbdkit-truncate-filter(1), nbdkit-xz-filter(1), virt-p2v(1).