Optimizing Costs for Stable Workload on Compute Engine Instances

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The best option to lower the costs for a business-critical workload on a fixed set of Compute Engine instances without any performance implications is to purchase Committed Use Discounts (CUD).

A Committed Use Discount is a way to reduce the cost of running Google Cloud resources, including Compute Engine instances, by making a commitment to use them for a specific term. By committing to use the instances for a certain period of time, the user is rewarded with a discounted rate for that instance type. This can result in cost savings of up to 57% compared to on-demand pricing.

Migrating the instances to a Managed Instance Group (MIG) is not the best option since it doesn't guarantee cost savings. MIGs are designed for autoscaling and managing groups of instances, which may not be necessary for a fixed workload. Additionally, the cost of a MIG may be higher than the cost of running individual instances.

Converting the instances to preemptible virtual machines is not recommended for business-critical workloads because these instances can be terminated at any time, usually after running for 24 hours. This can cause interruptions in the workload and result in data loss if the instance is not designed to handle abrupt termination.

Creating an Unmanaged Instance Group is also not recommended as it requires manual configuration of instances, making it difficult to manage and scale. It also does not provide cost savings compared to on-demand pricing.

In summary, purchasing Committed Use Discounts is the best option for lowering costs for a business-critical workload on a fixed set of Compute Engine instances without any performance implications.