Virtualization lets you ignore which physical server a VM runs on — until you can't. The hardware still sits beneath the abstraction, and its behavior leaks through. Two identically-specified cloud instances can perform very differently. The cause is shared tenancy: a given instance may sit on degraded hardware, or share a host with someone else's resource-hungry workload (the "noisy neighbor"). The API presents a uniform facade; the silicon underneath does not honor it. The practical consequence is that you cannot treat the cloud as infinite, uniform capacity. Reasoning about the physical layer pays off: - **Instance sizing** — some teams pick a VM size large enough to get a host entirely to themselves, even when they don't need the full capacity, to avoid noisy neighbors. - **Storage and networking** — the fastest option (e.g. local SSD) is not automatically the right one; it carries tradeoffs in portability, cost, and availability. This is mechanical sympathy applied to the cloud: the deeper your understanding down the stack into the hardware, the more reliably and efficiently your systems run. ## Related (Cross-Domain) - [[The Law of Leaky Abstractions - Joel Spolsky]] — the canonical statement of why this happens at all: no non-trivial abstraction, cloud or otherwise, fully hides what it's abstracting - [[LLM Mechanical Sympathy]] — the same "mechanical sympathy" concept applied to a different substrate: intuition for how a system behaves at its limits, built through operating it rather than building it - [[CPU Steal Time]] — the concrete, measurable signature of the exact noisy-neighbor mechanism this note describes - [[Migration as Remediation for Oversold Hosts]] — the practical response once hardware variability is detected: relocate rather than fight the contended host - [[Manufacturing is still pre AWS]] — cautionary precedent for a different domain's "AWS moment" aspiration: even the original AWS abstraction leaks physical reality through, so a manufacturing equivalent would inherit the same limit, not a clean abstraction --- *Source: [[Mechanical Sympathy and the Cloud — Kief Morris]] (Kief Morris, infrastructure-as-code.com, 23 Mar 2015)*