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instance_type is your cloud’s own name for a machine shape. Airship does not invent a size vocabulary of its own, so anything your account can launch, you can ask for.
Any EC2 instance type: c7i.xlarge, m8g.4xlarge, r7iz.metal-32xl, mac2-m2.metal, u7i-12tb.224xlarge.
A list tries each in order; see Fallbacks. That is the usual answer to a capacity shortage in one family.

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Architecture is inferred, not declared

There is no arch field. Airship reads the architecture from the instance type and picks the matching default image, runner build and driver path: The runner registers with arm64 or x64 as a label, so a job can ask for one specifically:
A custom image built for the wrong architecture is refused at planning time rather than failing at boot. If you keep per-architecture images, give each one its own definition.

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GPUs

gpu: means “this runner has a GPU”. It adds the gpu label, and on Ubuntu images that lack one it installs the NVIDIA driver. What the value means depends on the cloud:
The name is a flag — the instance type decides the hardware. Pick a GPU instance family and name the GPU you expect:
An image that already ships the driver — a deep-learning AMI, for instance — needs no installation; Airship detects one and leaves it alone.
GPU quotas are usually zero until you request an increase, and GPU stock is zonal — pin a zone so a shortage fails fast instead of walking a whole region. To turn a GPU off in a definition that inherits one, set gpu: false.

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Host maintenance

On Google Cloud, extra_config.scheduling decides what happens when the host your runner sits on needs maintenance:
MIGRATE is the default for machines that can do it. TERMINATE is the default — and the only option — for GPUs and spot, so MIGRATE together with preemptible or a GPU is a config error rather than a launch Compute Engine would refuse.

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How long a machine may live

Every machine schedules its own shutdown before anything else runs: eight hours by default. A runner that reaches it finishes as max_lifetime. A launched runner that no job arrives for within ten minutes finishes as idle — almost always a label mismatch.

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Choosing a size

Start one size up from habit

CI is usually bound by one slow step. Doubling the cores on a make -j build is often cheaper per job, not more expensive, because the machine lives for half as long.

Then fix the disk

A fast instance on a default volume is disk-bound. Raise throughput and IOPS before reaching for more cores. Disks

Spot for everything repeatable

With an on-demand fallback behind it, spot costs a retry now and then. Fallbacks

Arm where your toolchain allows

Graviton and Arm families are usually the best price per unit of work, and the runner labels let you split a matrix across both.