Instance File

Every VM instance stores its settings in instance.toml in its own directory, see File Locations. Cubic writes the file when the VM instance is created and updates it on every change.

Use cubic show to read the settings and cubic modify to change them. Editing the file by hand works but is not checked, and a change only takes effect after a restart. Edit it while the VM instance is stopped, because Cubic rewrites the file on its own, for example when it assigns a port at the start.

Fields

Field	Value	Description
`arch`	`"AMD64"` or `"ARM64"`	Architecture of the guest. Set when the VM instance is created and never changed afterwards.
`user`	user name	User account inside the guest. cloud-init created it on the first boot, so a later change locks you out.
`cpus`	number	vCPUs handed to QEMU.
`mem`	bytes	Memory of the guest, in bytes rather than a size string.
`disk_capacity`	bytes	Size the disk image may grow to, in bytes. A disk can only grow.
`ssh_port`	port	Host port forwarded to port 22 of the guest. Assigned when the VM instance is created. Cubic assigns a new one when the port is taken at the next start.
`monitor_port`	port	Host port of the QEMU monitor. Assigned at every start, so it changes from run to run.
`console_port`	port	Host port of the serial console. Assigned at every start as well.
`hostfwd`	list of strings	Forwarded ports in QEMU notation, written as `protocol:host_ip:host_port-:guest_port`. `cubic modify --port` takes the shorter `host:guest` form instead.
`execute`	shell command	Commands the guest runs once on its first boot. Several `--exec` arguments are joined with `&&`.
`isolate`	`true` or `false`	Cuts the guest off the network.
`ssh_host_key`	public key	SSH host key of the guest. Cubic pins it on the first connect and asks before it accepts a different key later.

arch, user, cpus, mem, disk_capacity and ssh_port are always written. The rest is left out while it is unset.

Example

arch = "AMD64"
user = "john"
cpus = 4
mem = 4294967296
disk_capacity = 107374182400
ssh_port = 37635
monitor_port = 42661
console_port = 37913
hostfwd = ["tcp:127.0.0.1:8080-:80"]
execute = "sudo apt update && sudo apt install -y nginx"
isolate = false
ssh_host_key = "ssh-ed25519 AAAA..."

Cubic Never Stores

Some state lives elsewhere and is not part of the file:

Snapshots are read back from the disk image itself.
The disk usage is measured on the fly.
Whether the VM instance runs follows from qemu.pid.
Image tags are derived from the image list at every read.