AMI

An Amazon Machine Image (AMI) is the template an EC2 instance is launched from. It bundles one or more EBS snapshots (Elastic Block Store, the persistent disk service EC2 instances boot from) containing the root filesystem, together with the metadata EC2 needs to boot it: architecture, virtualization type, root device mapping, and the boot mode — legacy BIOS or UEFI, the modern replacement for it.

An AMI is regional. An image registered in us-east-1 cannot be launched in eu-west-1 until it has been copied there, and the copy gets a different AMI ID. Anything that references AMIs across regions — a Terraform module, a launch template, an autoscaling group — has to account for that, which is why a fleet spanning regions is usually deploying several AMI IDs that are the same image.

AMIs are also private by default, and sharing one means explicitly granting launch permission to another AWS account or making it public.

Creating an AMI from an image built elsewhere means importing a disk. AWS accepts RAW (an unstructured, byte-for-byte disk dump), VHD (Virtual Hard Disk, Microsoft’s format), and VMDK (Virtual Machine Disk, VMware’s) for import — but not QCOW2, the format the Linux virtualization stack most commonly builds in. A conversion step is therefore unavoidable when a golden image is built under QEMU/KVM — the open-source virtualization stack most Linux image builds run on — and published to EC2.