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

Provided by: nbdkit_1.40.4-5ubuntu1_amd64

NAME

       nbdkit-ondemand-plugin - create filesystems on demand

SYNOPSIS

        nbdkit ondemand dir=EXPORTSDIR [size=]SIZE [wait=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 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.

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 (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.

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" and "EXAMPLES" sections 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.

[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).

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.

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

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