Compute
What to know
Compute choices start with shape: standard (general purpose), flexible (choose OCPUs and memory within a range), DenseIO (local NVMe), GPU, and Ampere A1 (Arm). On x86, one OCPU is one physical core exposed as two vCPUs; on Arm A1, one OCPU equals one vCPU - a classic exam trap. Instances can be virtual machines, bare metal, or run on dedicated hosts, and they live in a compartment, an availability domain, and a fault domain.
Supporting concepts include images (platform, custom, marketplace), boot volumes, instance metadata and user data for cloud-init, instance principals for calling OCI APIs without storing keys, instance pools with autoscaling policies, and capacity types (on-demand, preemptible, reserved). Preemptible instances are cheap but can be reclaimed, so they suit fault-tolerant batch work.
Traps
- Mixing OCPU and vCPU counts when sizing or comparing prices.
- Assuming a shape can be resized without a reboot, or that every shape exists in every availability domain.
- Treating preemptible capacity as durable.
- Forgetting that an instance principal needs a dynamic group plus a policy before it can call services.
Rule of thumb
Pick a flexible shape when utilization is uncertain, and size by measured OCPU and memory rather than by matching another provider's instance names. For stateless or restartable workloads, preemptible capacity inside an instance pool gives the best cost per unit of work.