Many organizations choose a VM SKU and version (e.g., `D4s_v3`) during the initial planning phase of a project, often based on availability, compatibility, or early cost estimates. Over time, Microsoft releases newer hardware generations (e.g., `D4s_v4`, `D4s_v5`) that offer equivalent or better performance at the same or reduced cost. However, existing VMs are not automatically migrated, and these newer versions are often overlooked unless intentionally evaluated.
This inefficiency tends to persist because switching to a newer version typically requires VM deallocation and resizing, which introduces temporary downtime. As a result, outdated VM series versions continue to run indefinitely, even in environments where brief downtime is acceptable. The cost delta between series versions—especially across certain families like `D`, `E`, or `F`—can be significant when scaled across environments or multiple VMs. Note that VM series versions (v3, v4, v5) are distinct from VM generations (Gen 1 vs Gen 2), with series versions representing the primary opportunity for cost optimization.
Azure VMs are billed per second based on instance size, operating system, and hardware generation (series version). Newer VM versions typically offer equivalent or better performance at the same of reduced cost due to improved infrastructure efficiency.. Legacy versions may persist in production environments despite suboptimal price/performance ratios, simply because they were originally selected during initial provisioning or project planning.
