When modifying AWS RDS storage allocation (for either standard or Provisioned IOPS storage types), the actual downtime experienced depends on several factors:

# Example AWS CLI command for storage modification
aws rds modify-db-instance \
    --db-instance-identifier mydbinstance \
    --allocated-storage 500 \  # New storage size in GB
    --apply-immediately

Based on AWS documentation and empirical observations:

• Most RDS engines (MySQL, PostgreSQL, Aurora): Experience near-zero downtime (under 30 seconds)
• SQL Server (pre-November 2017 configurations): May experience 2-5 minutes downtime
• Storage-optimization phase: While marked as "Storage-optimization", the instance remains fully operational

To ensure minimal impact during storage scaling:

# CloudFormation template snippet for controlled storage modification
Resources:
  MyDB:
    Type: AWS::RDS::DBInstance
    Properties:
      AllocatedStorage: !Ref StorageSize
      StorageType: gp2
      AllowMajorVersionUpgrade: false
      AutoMinorVersionUpgrade: true

Use these AWS CLI commands to track progress:

# Check instance status
aws rds describe-db-instances \
    --db-instance-identifier mydbinstance \
    --query 'DBInstances[0].DBInstanceStatus'

# Check storage optimization status
aws rds describe-db-instances \
    --db-instance-identifier mydbinstance \
    --query 'DBInstances[0].StatusInfos'

For legacy SQL Server instances, consider these additional steps:

# PowerShell script to automate failover during maintenance
Import-Module AWSPowerShell
$instance = Get-RDSDBInstance -DBInstanceIdentifier "mssql-instance"
if ($instance.EngineVersion -lt "14.00") {
    Write-Host "Legacy SQL Server detected - scheduling extended maintenance window"
    # Implement custom failover logic here
}

Important constraints to note:

• 6-hour cooldown period between storage modifications
• Maximum storage increase per modification: 10TiB (varies by engine)
• Storage decreases are not supported

While AWS states performance shouldn't degrade during storage optimization, monitor these metrics:

# CloudWatch metrics to watch during modification
aws cloudwatch get-metric-statistics \
    --namespace AWS/RDS \
    --metric-name CPUUtilization \
    --dimensions Name=DBInstanceIdentifier,Value=mydbinstance \
    --start-time $(date -d "1 hour ago" +%FT%T) \
    --end-time $(date +%FT%T) \
    --period 60 \
    --statistics Average

When modifying storage for Amazon RDS instances, the behavior varies significantly depending on:

• Database engine (MySQL, PostgreSQL, SQL Server, etc.)
• Storage type transition (gp2 to io1 or vice versa)
• Whether storage optimization has occurred since November 2017

For most modern RDS configurations (post-2017 optimizations), storage scaling operates as follows:

# AWS CLI command to modify storage
aws rds modify-db-instance \
    --db-instance-identifier your-instance \
    --allocated-storage 500 \
    --apply-immediately

Key observations from production environments:

• MySQL/PostgreSQL: Typically experience 10-30 seconds of latency during the modification
• SQL Server (legacy): May incur 2-5 minutes downtime during storage-optimization phase
• Multi-AZ deployments: Often show better availability during modifications

Use this CloudWatch metrics query to track storage modifications:

aws cloudwatch get-metric-data \
    --metric-data-queries file://query.json \
    --start-time $(date -v-1H +%s) \
    --end-time $(date +%s)

Where query.json contains:

{
  "Id": "storageOpt",
  "MetricStat": {
    "Metric": {
      "Namespace": "AWS/RDS",
      "MetricName": "StorageOptimization",
      "Dimensions": [
        {
          "Name": "DBInstanceIdentifier",
          "Value": "your-instance"
        }
      ]
    },
    "Period": 60,
    "Stat": "Average"
  },
  "ReturnData": true
}

Based on AWS Premium Support cases and community reports:

• Always initiate changes during maintenance windows
• For SQL Server, expect longer optimization times with larger storage increments
• Storage modifications are blocking operations - no further changes allowed for 6 hours

Here's a Lambda function template for automated storage scaling:

import boto3

def lambda_handler(event, context):
    rds = boto3.client('rds')
    
    try:
        response = rds.modify_db_instance(
            DBInstanceIdentifier=event['DBInstanceIdentifier'],
            AllocatedStorage=event['NewStorageSize'],
            ApplyImmediately=True
        )
        return {
            'statusCode': 200,
            'body': response
        }
    except Exception as e:
        return {
            'statusCode': 500,
            'error': str(e)
        }