/amazon-aurora-mysql

Aurora serverless — Core Concepts

Aurora Capacity Units (ACU)

• 1 ACU ≈ 2 GiB memory + corresponding CPU and networking
• Range: 0.5 to 256 ACU per instance, in 0.5 ACU increments (min/max configured at the cluster level; each instance scales independently within that range — a 3-instance cluster can consume up to 768 ACU total)
• Available for the Aurora MySQL-Compatible Edition

Scaling Behavior

• Scales continuously (not in steps) based on CPU, connections, and available memory
• Scale-up: near-instant (seconds), no connection disruption
• Scale-down: continuous and granular (capacity re-evaluated every second; scales down when current capacity exceeds load). The scale-down rate is governed by current capacity, not a fixed cooldown — no "~15 min cooldown" applies to Aurora serverless (v2). (The 15-min cooldown belonged to the deprecated Aurora Serverless v1.) Certain features (global databases, Performance Insights, Enhanced Monitoring, CloudWatch Logs export, Advanced Auditing / server_audit, elevated max_connections) can hold capacity above minimum.

Scale-to-Zero (Auto-Pause)

Supported versions:

• Aurora MySQL: 3.08.0+

Incompatible with: RDS Proxy, binary logging (binlog) enabled, Global Database (primary), Zero-ETL

Trigger: 0 user connections for the configured timeout. Aurora background processes keep CPU at ~8-10% even when idle — this is normal and does not prevent pause.

Resume latency: ~15s if paused <24h, ~30s if paused >24h.

ACU Sizing from Provisioned Instances

weighted_cpu = (P95_CPU × 0.95 + Max_CPU × 0.05) / 100
raw_acu = weighted_cpu × vCPU_count × family_ratio
estimated_acu = round_up_to_nearest_0.5(raw_acu)

Family ratios (ACU per vCPU):

Min/Max ACU Configuration

Minimum ACU — covers:

1. Average CPU load (prevents scaling churn)
2. Connection memory floor: ~10 MB per connection → 100 connections ≈ 0.5 ACU
3. Working set floor (advisory): 1 GiB working set ≈ 0.5 ACU. Setting min below this trades cost for occasional I/O latency spikes on scale-up.

Formula: min_acu = MAX(0.5, avg_cpu_acu, connection_acu_floor)

Maximum ACU — covers peaks with headroom (per instance):

• max_acu = MIN(peak_acu × 1.3, 256)
• Ensure max ≥ typical × 1.5 for burst capacity
• If per-instance peak exceeds 256 ACU, workload exceeds serverless capacity on a single instance
• Total cluster peak ACU = per-instance peak × number of instances

Pricing (us-east-1)

Monthly cost:

compute = estimated_acu × $0.12/ACU-Hr × 730 hours
storage = storage_gib × $0.10/GiB-month   (Standard; × $0.225/GiB-month for I/O-Optimized)
monthly = compute + storage

Commitment discounts: Database Savings Plans (1-year) cover serverless ACU. Reserved Instances do NOT apply to serverless.