nbdkit-file-plugin - nbdkit file plugin
nbdkit file [file=]FILENAME
[cache=default|none] [fadvise=normal|random|sequential] nbdkit file dir=DIRECTORY nbdkit file fd=FILE_DESCRIPTOR nbdkit file dirfd=FILE_DESCRIPTORnbdkit-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.
The file is writable unless either the -r command line option, or nbdkit-readonly-filter(1) is used.
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.
(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.
(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.
See "Serving multiple files and block devices" below.
(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.
See "Serving multiple files and block devices" below.
(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.
(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.
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).
(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.
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/exportswill serve three exports called "disk1", "disk2" and "sda1". The first two are regular files and the last is a block device. You can add or remove files or symbolic links from the directory while nbdkit 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=disk1If 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=sequentialAs described in the "PARAMETERS" section above, on Linux this disables or increases the amount of read-ahead that the kernel does.
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=noneOnly use fadvise=sequential if reading, and the reads are mainly sequential.
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.
You can obtain extra information about how the file plugin was compiled by doing:
nbdkit file --dump-pluginSome 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=yesIf set, the plugin has support for getting the minimum and preferred I/O size of block devices.
file_blkrotational=yesIf set, the plugin has support for getting the rotational property of block devices.
file_blksszget=yesfile_blkzeroout=yesIf 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=yesIf set, the plugin can read file extents.
file_falloc_fl_punch_hole=yesIf set, the plugin may be able to punch holes (make sparse) files and block devices.
file_falloc_fl_zero_range=yesIf set, the plugin may be able to efficiently zero ranges of files and block devices.
winfile=yesIf present, this is the Windows version of the file plugin with reduced functionality and some special Windows-only features, as noted in this manual.
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.
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.
To concatenate and export multiple files, use nbdkit-split-plugin(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.
The plugin.
Use nbdkit --dump-config to find the location of $plugindir.
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-luks-filter(1), nbdkit-noextents-filter(1), nbdkit-readonly-filter(1), nbdinfo(1).
Eric Blake
Nir Soffer
Richard W.M. Jones
Copyright Red Hat
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