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

nbdkit-ondemand-plugin - create filesystems on demand

 nbdkit ondemand dir=EXPORTSDIR [size=]SIZE [wait=true]
                 { [type=ext4|xfs|vfat|...] [label=LABEL]
                   | command=COMMAND [VAR=VALUE ...] }

This is a plugin for nbdkit(1) which creates persistent filesystems on demand. Clients may simply connect to the server, requesting a particular export name, and a new filesystem is created if it does not exist already. Clients can also disconnect and reconnect with the same export name and the same filesystem will still be available. Filesystems are stored in a directory on the server, so they also persist over nbdkit and server restarts.

Each filesystem is locked while it is in use by a client, preventing two clients from accessing the same filesystem (which would cause corruption).

Similar plugins include nbdkit-file-plugin(1) which can serve a predefined set of exports (clients cannot create more), nbdkit-tmpdisk-plugin(1) which creates a fresh temporary filesystem for each client, and nbdkit-linuxdisk-plugin(1) which exports a single filesystem from a local directory on the server.

When a new export name is requested by a client, a sparse file of the same name is created in "dir=EXPORTSDIR" on the server. The file will be formatted with mkfs(8). The size of the file is currently fixed by the "size=SIZE" parameter, but we intend to make this client-configurable in future. The filesystem type and label may also be specified, otherwise "ext4" and no label is used.

Export names must be ≤ "NAME_MAX" (usually 255) bytes in length and must not contain certain characters including ".", "/" and ":". There may be other limitations added in future. Client requests which do not obey these restrictions are rejected. As a special case, export name "" is mapped to the file name default.

You should only use this in an environment where you trust all your clients, since clients can use this plugin to consume arbitrary amounts of disk space by creating unlimited exports. It is therefore best to take steps to limit where clients can connect from using nbdkit-ip-filter(1), firewalls, or TLS client certificates.

Instead of running mkfs you can run an arbitrary command (a shell script fragment) to create the disk.

The other parameters to the plugin are turned into shell variables passed to the command. For example "type" becomes the shell variable $type, etc. Any parameters you want can be passed to the plugin and will be turned into shell variables (not only "type" and "label") making this a very flexible method to create filesystems and disks of all kinds.

Two special variables are also passed to the shell script fragment:

$disk
The absolute path of the disk file. This is partially controlled by the client so you should quote it carefully. This file is not pre-created, the command must create it for example using: 

 truncate -s $size "$disk"
$size
The virtual size in bytes. This is the "size" parameter, converted to bytes. Note the final size served to the client is whatever disk size "command" creates.

Run the server like this:

 mkdir /var/tmp/exports
 nbdkit ondemand dir=/var/tmp/exports 1G

Clients can connect and create 1G ext4 filesystems on demand using commands such as these (note the different export names):

 nbd-client server /dev/nbd0 -N export1
 mount /dev/nbd0 /mnt


 guestfish --format=raw -a nbd://localhost/export2 -m /dev/sda


 qemu-img info nbd:localhost:10809:exportname=export2

On the server you would see two filesystems created:

 $ ls -l /var/tmp/exports
 -rw-rw-r--. 1 rjones rjones 1073741824 Aug 13 21:40 export1
 -rw-rw-r--. 1 rjones rjones 1073741824 Aug 13 21:40 export2

The plugin does not clean these up. If they are no longer needed then the server admin should delete them (or use a tmp cleaner).

Instead of running mkfs(8) to create the initial filesystem, run "COMMAND" (a shell script fragment which usually must be quoted to protect it from the shell). See "The command parameter" and "EXAMPLES" sections above.
The directory where filesystems are saved. When first using this plugin you should point this to an empty directory. When clients connect, filesystems are created here.

This parameter is required.

Select the filesystem label. The default is not set.
[size=]SIZE
Specify the virtual size of all of the filesystems.

If using "command", this is only a suggested size. The actual size of the resulting disk will be the size of the disk created by "command".

This parameter is required.

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

Select the filesystem type. The default is "ext4". Most non-networked, non-cluster filesystem types supported by the mkfs(8) command can be used here.
If set, if two clients try to connect at the same time to the same export then the second client will wait for the first to disconnect. The default behaviour is to reject the second client with the error message: 

 filesystem is locked by another client

This setting is sometimes useful if you are making repeated connections and at the network level the first connection does not fully disconnect before the next connection starts. This can also happen as a side-effect of using "guestfish --ro" which opens two NBD connections in quick succession.

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

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

"nbdkit-ondemand-plugin" first appeared in nbdkit 1.22.

nbdkit(1), nbdkit-plugin(3), nbdkit-file-plugin(1), nbdkit-ip-filter(1), nbdkit-limit-filter(1), nbdkit-linuxdisk-plugin(1), nbdkit-memory-plugin(1), nbdkit-tmpdisk-plugin(1), nbdkit-tls(1), mkfs(8), mke2fs(8).

Richard W.M. Jones

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2023-01-04 nbdkit-1.32.5