arjunprabhulal Rightsizing
Most instance sizes are set once, at launch, from a guess made before real traffic existed — and then never revisited, because a running system that isn't on fire doesn't generate a reason to go back and check. The gap between provisioned and used capacity grows quietly for as long as nobody looks, and by the time someone does look it's often large enough to embarrass whoever picked the original size.
Rightsizing is not a one-time correction. Usage changes with every feature release, traffic pattern shift, and dependency change, so a size that was correct six months ago is a guess again today. The discipline is the measurement loop, not the resize itself.
Size from what the system actually uses, not from what someone guessed it might need.
1. Size from measured usage, never from instinct
"This feels like it needs 4 vCPUs" is not data. Pull actual CPU, memory, disk, and network utilization over a representative window before changing anything — a resize based on a hunch is just as likely to be wrong in the expensive direction as the cheap one.
- Use a window that includes peak traffic, not just a quiet afternoon that understates real need.
- Check all resource dimensions, not just CPU — a service throttled on memory or IOPS won't show up in a CPU-only view.
Done when: every resize decision cites a metrics window, not a guess.
2. Look at the right percentile, not the average
Average utilization hides the peaks that actually determine whether a resource is adequately sized — a workload that averages 20% CPU but spikes to 95% during checkout on Fridays will fall over if sized to the average. Size to a high percentile (p95 or p99) of the metric that matters for the workload's actual failure mode, with headroom for the spike, not to what it looks like on a typical Tuesday.
Done when: the sizing decision states which percentile was used and why it fits the workload's traffic shape.
3. Prefer autoscaling to a fixed guess wherever demand varies
A fixed size is a bet that usage stays flat — true for very few real workloads. Where demand
varies meaningfully by time of day, day of week, or event, autoscaling adapts capacity to the
actual curve instead of provisioning for the peak all day and wasting the difference the rest of
the time. See autoscaling for the mechanics of scaling policies and cooldowns.
- Fixed capacity fits workloads with genuinely flat, predictable demand — a fixed-size batch cluster with a known job size, for instance.
- Autoscaling fits everything else — anything with a daily cycle, a weekly cycle, or bursty traffic.
Done when: every workload with variable demand runs on autoscaling instead of a fixed size picked to cover its peak.
4. Treat downsizing as a queue, not a one-time sweep
A single rightsizing sweep finds the backlog that accumulated before anyone looked, but usage keeps drifting the moment the sweep ends. Turning rightsizing into a standing queue — new candidates surfaced as usage patterns change, reviewed and actioned on a cadence — keeps the gap from re-accumulating into the next big embarrassing sweep.
Done when: rightsizing candidates are reviewed on a recurring cadence, not discovered fresh each time someone asks why the bill is high.
5. Treat CPU and memory as separate sizing problems
A resource can be oversized on CPU and undersized on memory at the same time, and picking a size that only looks at one dimension will get the other wrong. Size each resource dimension against its own utilization data, then pick the smallest instance type or request/limit pair that covers all of them with headroom — not the smallest that covers whichever dimension was checked first.
Done when: every sizing decision states the CPU, memory, and (where relevant) storage or IOPS utilization it was based on, not just one dimension.
6. Re-measure after every change
A resize changes the workload's behavior — a smaller instance under real load may show different utilization than the old one did, because contention, garbage collection, or I/O wait patterns shift with the resource ceiling. Re-measuring after the change, not just before it, catches a resize that was too aggressive before it turns into an incident.
Done when: every resize is followed by a post-change metrics check confirming the new size holds up under real traffic.
Report
State which resources were resized, the percentile and window the sizing was based on, and how much provisioned capacity was removed. Name the honest gap — usually a workload whose traffic pattern is too irregular to trust a percentile-based estimate yet, or one still pending autoscaling migration — rather than claiming the whole fleet is now optimally sized.