Ubuntu Manpage: nbdkit-ondemand-plugin - create filesystems on demand

Provided by: nbdkit_1.42.4-1ubuntu1_amd64

NAME

       nbdkit-ondemand-plugin - create filesystems on demand

SYNOPSIS

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

DESCRIPTION

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 persist
over server restarts.

By default each export 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.

Export names
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 "." at the start, 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.

Security considerations
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.

The command parameter
Instead of running mkfs you can run an arbitrary command or shell script 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.

Dead client detection
Clients which "go away" without disconnecting (such as clients which reboot) may cause exports to remain
locked when they are no longer used. Practically this can cause problems such as the same client being
unable to reconnect after a reboot. To avoid this you should enable keepalives to detect dead clients.
Use the nbdkit --keepalive option, or for more fine-grained control see the discussion of keepalives in
nbdkit-service(1).

EXAMPLE

       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).

PARAMETERS

command='COMMAND'
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" above.

dir=EXPORTSDIR
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.

label=LABEL
Select the filesystem label. The default is not set.

share=true
If set, the export is not locked. Two or more clients may connect to the same export. This usually
results in filesystem corruption (if using a regular filesystem like ext4), so you have to use some
other means to guarantee exclusive access or to ensure sharing is safe.

[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=" prefix may be omitted in most cases. See "Magic parameters" in nbdkit(1).

type=FS
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.

wait=true
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.

If "share=true" then locking is not used and the "wait" parameter has no effect.

FILES

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

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

VERSION

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

AUTHORS

       Richard W.M. Jones

COPYRIGHT

       Copyright Red Hat

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 BE LIABLE FOR ANY DIRECT, INDIRECT,
       INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,  PROCUREMENT  OF
       SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
       ON  ANY  THEORY  OF  LIABILITY,  WHETHER  IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR
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       DAMAGE.

nbdkit-1.42.4                                      2025-06-16                          nbdkit-ondemand-plugin(1)