nbdkit-ondemand-plugin - Man Page

 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.

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).

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.

$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

Copyright Red Hat

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nbdkit(1), nbdkit-plugin(3), nbdkit-release-notes-1.22(1), nbdkit-release-notes-1.26(1), nbdkit-tmpdisk-plugin(1).