nbdkit-tmpdisk-plugin1

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 temporary disks of all kinds.

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

$disk

The absolute path of the disk file. Note that this is not pre-created, you must create it yourself, for example using:

 truncate -s $size "$disk"

$disk is deleted automatically when the client disconnects.

$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.

Security considerations

Because each client gets a new disk, the amount of disk space required on the server can be as much as number of clients × size. It is therefore best to limit the number of clients using nbdkit-limit-filter(1) or take steps to limit where clients can connect from using nbdkit-ip-filter(1), firewalls, or TLS client certificates.

Remote tmpfs

One use for this is to create a kind of "remote tmpfs(5)" for thin clients. On the server you would run (see nbdkit-service(1)):

 nbdkit tmpdisk 16G

To set up each thin client follow the instructions in nbdkit-client(1). Clients will see a fresh, empty, mounted directory after boot.

Overriding mkfs options

Using "command" allows you to easily override any mkfs option, for example:

 nbdkit tmpdisk 16G command='
     truncate -s $size "$disk"
     mke2fs -F -N 10000 -t ext4 "$disk"
 '

Serve a fresh blank disk to each client

Again using "command", this demonstrates serving any file that you can create locally to the client. This is different from nbdkit-memory-plugin(1) because the clients all see their own private disk (instead of all seeing the same shared disk):

 nbdkit tmpdisk 16G command=' truncate -s $size "$disk" '

Serve a fresh operating system to each client

 nbdkit tmpdisk 16G os=fedora-31 \
     command=' virt-builder -o "$disk" --size ${size}b "$os" '

Serve a throwaway snapshot of a base image to each client

You could create a base image using mke2fs(8) -d option, virt-builder(1), or similar tools. Then in the command you could copy and serve different throwaway snapshots to each client:

 truncate -s 1G base.img
 mke2fs -d initial-content/ -F -t ext4 base.img
 nbdkit tmpdisk size=0 base=$PWD/base.img \
     command=' cp --sparse=always "$base" "$disk" '

The unusual "size=0" parameter is because when using "command", "size" is only a request (but the parameter is required). In this case the command ignores the requested size. The final size is the size of $disk created by the cp(1) command.