DOKK / manpages / debian 12 / nbdkit / nbdkit-linuxdisk-plugin.1.en
nbdkit-linuxdisk-plugin(1) NBDKIT nbdkit-linuxdisk-plugin(1)

nbdkit-linuxdisk-plugin - create virtual Linux disk from directory

 nbdkit linuxdisk [dir=]DIRECTORY
                  [label=LABEL] [type=ext2|ext3|ext4]
                  [size=[+]SIZE]

"nbdkit-linuxdisk-plugin" is a plugin for nbdkit(1) which creates an ext2-, ext3- or ext4-formatted disk image from a directory on the fly. The files in the specified directory (and subdirectories) appear in the virtual disk, which is served read-only over the NBD protocol.

The virtual disk is partitioned with a single GPT partition containing the filesystem.

The virtual disk can be used as a Linux root (or other) filesystem. Most features of Linux filesystems are supported, such as hard links, symbolic links, block special devices etc.

To create a FAT-formatted virtual floppy disk, see nbdkit-floppy-plugin(1). To create a CD/ISO, see nbdkit-iso-plugin(1). To create an empty filesystem for each client that connects (like a "remote tmpfs") use nbdkit-tmpdisk-plugin(1).

Create a virtual disk, giving it a filesystem label. Note that clients will not be able to modify the filesystem, so it is safe to share it with multiple clients.
Add a writable overlay (see nbdkit-cow-filter(1), allowing the disk to be written by the client. Multiple clients must not be allowed to connect at the same time (even if they all mount it read-only) as this will cause disk corruption.
The same but specifying that at least 1G of free space should be available in the filesystem (not including the space taken by the initial filesystem).
Instead of serving a partitioned disk image, serve just the "naked" filesystem (ie. the first partition, see nbdkit-partition-filter(1)).
This serves nothing. Instead it turns a directory into a disk image, writing it to ext2fs.img (see nbdkit-captive(1)). The resulting image is a partitioned disk.
This creates and boots a minimal busybox(1)-based virtual appliance. This assumes that your kernel (/boot/vmlinuz) contains the ext2 or ext4 driver compiled in, but most Linux distro kernels have that. 

 mkdir -p root/{bin,dev,proc,sbin,sys,usr/bin,usr/sbin}
 sudo mknod root/dev/console c 5 1
 cp /sbin/busybox root/sbin/
 ln root/sbin/busybox root/sbin/init
 ln root/sbin/busybox root/bin/ls
 ln root/sbin/busybox root/bin/sh
 nbdkit -U - linuxdisk root --run '
   qemu-kvm -display none -kernel /boot/vmlinuz -drive file=nbd:unix:$unixsocket,snapshot=on -append "console=ttyS0 root=/dev/sda1 rw" -serial stdio
 '

You can drop any extra files you need into the root/ directory and they will be copied into the appliance before boot. After booting type these commands to complete the environmental setup:

 /sbin/busybox --install
 mount -t proc proc /proc
 mount -t sysfs sys /sys

[dir=]DIRECTORY
Specify the directory containing files and subdirectories which will be added to the virtual disk. Files inside this directory will appear in the root directory of the virtual disk.

This parameter is required.

"dir=" is a magic config key and may be omitted in most cases. See "Magic parameters" in nbdkit(1).

The optional label for the filesystem.
The total (virtual) size of the filesystem.

If the "size" parameter is omitted the plugin will try to size the filesystem with just enough space to contain the files and directories that are initially loaded, and there will not be much extra space.

Using "size=SIZE" specifies the required virtual size of the whole filesystem (including initial files and extra space). If this is set too small for the initial filesystem then the plugin will fail to start.

Using "size=+SIZE" specifies the minimum free space required after the initial filesystem has been loaded. (The actual free space might be slightly larger).

Select the filesystem type. The default is "ext2".

The original file UIDs and GIDs are recreated as far as possible. Note that UIDs/GIDs will likely map to different users and groups when read by a virtual machine or other NBD client machine.

"TMPDIR"
The filesystem image is stored in a temporary file located in /var/tmp by default. You can override this location by setting the "TMPDIR" environment variable before starting nbdkit.

$plugindir/nbdkit-linuxdisk-plugin.so
The plugin.

Use "nbdkit --dump-config" to find the location of $plugindir.

"nbdkit-linuxdisk-plugin" first appeared in nbdkit 1.12.

mke2fs(8), nbdkit(1), nbdkit-plugin(3), nbdkit-captive(1), nbdkit-cow-filter(1), nbdkit-file-plugin(1), nbdkit-floppy-plugin(1), nbdkit-iso-plugin(1), nbdkit-partition-filter(1), nbdkit-partitioning-plugin(1), nbdkit-tmpdisk-plugin(1),

Richard W.M. Jones

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2023-01-04 nbdkit-1.32.5