RDS Cost Spike with Multi-AZ (200% increase)

Bill Spike Diagnosis: RDS

Initial Assessment

• Previous bill: $150/month
• Current bill: $450/month
• Increase: $300 (200% increase)
• Instance type: db.t3.medium PostgreSQL
• Recent change: Multi-AZ enabled last week (user reports)

RDS Cost Components for db.t3.medium PostgreSQL

Analysis: Your 3x increase ($150 -> $450) suggests Multi-AZ is absolutely the cause, combined with one or more secondary factors.

Investigation Steps

Step 1: Verify Multi-AZ Was Recently Enabled

aws rds describe-events \
  --source-type db-instance \
  --source-identifier your-db-name \
  --start-time $(date -u -d '30 days ago' +%Y-%m-%dT%H:%M:%S) \
  --duration 43200 \
  --query 'Events[?contains(Message, `Multi-AZ`) || contains(Message, `storage`) || contains(Message, `instance class`)]'

Look for events like:

• “Multi-AZ instance modification complete”
• “Converted from single-AZ to Multi-AZ”

Step 2: Check Current RDS Configuration

aws rds describe-db-instances \
  --db-instance-identifier your-db-name \
  --query 'DBInstances[0].{
    InstanceClass:DBInstanceClass,
    MultiAZ:MultiAZ,
    Storage:AllocatedStorage,
    StorageType:StorageType,
    EngineVersion:EngineVersion,
    BackupRetentionPeriod:BackupRetentionPeriod
  }' \
  --output table

Expected output shows MultiAZ: True if enabled.

Step 3: Check PostgreSQL Version for Extended Support Fees

If your PostgreSQL version is 11 or earlier, you’re paying Extended Support fees:

aws rds describe-db-instances \
  --db-instance-identifier your-db-name \
  --query 'DBInstances[0].EngineVersion'

Likely Root Causes (Ranked by Probability)

1. Multi-AZ Enabled (95% probability) - PRIMARY CAUSE

Why this fits:

• You explicitly stated Multi-AZ was enabled last week
• Multi-AZ doubles both instance hours AND storage costs
• db.t3.medium Single-AZ: ~$52-80/month -> Multi-AZ: ~$104-160/month (instance only)

Cost impact: Multi-AZ doubles:

• Instance hours: $52-80/month -> $104-160/month (+$52-80)
• Storage (assuming 200GB): $23/month -> $46/month (+$23)
• Total Multi-AZ impact alone: ~$75-103/month increase

2. Storage Auto-Scaling Triggered (60% probability) - LIKELY CONTRIBUTING

Why this fits:

• Multi-AZ alone explains ~$75-103 increase, but you saw $300 increase
• Storage may have auto-scaled from 100GB to 300-400GB
• With Multi-AZ, storage costs are doubled

3. PostgreSQL Extended Support Fees (40% probability)

If running PostgreSQL 11 or earlier:

• db.t3.medium Extended Support: $0.036/hour ($26/month)
• With Multi-AZ, this could be ~$52/month

Resolution Options

Option A: Keep Multi-AZ, Optimize Other Costs (Recommended for Production)

1. Upgrade PostgreSQL version (if running 11 or earlier):

# Take snapshot first
aws rds create-db-snapshot \
  --db-instance-identifier your-db-name \
  --db-snapshot-identifier pre-upgrade-snapshot-$(date +%Y%m%d)

# Upgrade to PostgreSQL 12+
aws rds modify-db-instance \
  --db-instance-identifier your-db-name \
  --engine-version 12.19 \
  --apply-immediately

Savings: ~$26-52/month

2. Cap storage auto-scaling:

aws rds modify-db-instance \
  --db-instance-identifier your-db-name \
  --max-allocated-storage 250

Option B: Disable Multi-AZ (If High Availability Not Required)

aws rds modify-db-instance \
  --db-instance-identifier your-db-name \
  --no-multi-az \
  --apply-immediately

# WARNING: Causes 1-2 minutes downtime
# WARNING: Schedule during off-peak hours
# WARNING: No automatic failover after this

Savings: ~$75-180/month
New expected bill: ~$170-270/month

Option C: Purchase Reserved Instance (If Keeping Current Setup)

# Find available 1-year RI offerings
aws rds describe-reserved-db-instances-offerings \
  --db-instance-class db.t3.medium \
  --product-description postgresql \
  --multi-az \
  --duration 31536000 \
  --offering-type "No Upfront"

Savings: ~$180-270/month (40-60% off On-Demand)

Prevention Measures

Immediate:

aws cloudwatch put-metric-alarm \
  --alarm-name RDS-Monthly-Cost-Alert \
  --alarm-description "Alert when RDS costs exceed $200" \
  --metric-name EstimatedCharges \
  --namespace AWS/Billing \
  --statistic Maximum \
  --period 86400 \
  --evaluation-periods 1 \
  --threshold 200 \
  --comparison-operator GreaterThanThreshold \
  --dimensions Name=ServiceName,Value=AmazonRDS \
  --alarm-actions arn:aws:sns:us-east-1:123456789012:billing-alerts

What NOT to Do

1. Don’t panic and immediately disable Multi-AZ without understanding HA requirements - Multi-AZ provides automatic failover
2. Don’t try to reduce allocated storage - AWS does not support shrinking RDS storage
3. Don’t delete automated backups - Essential for disaster recovery, minimal cost savings
4. Don’t upgrade PostgreSQL in production without testing - Always test in staging first
5. Don’t change multiple settings at once - Isolate changes to identify impact