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Elastic Load Balancing (ELB) should be used to horizontally scale Amazon EC2 instances by distributing incoming traffic across multiple instances, improving availability and fault tolerance. ELB works in conjunction with Auto Scaling groups to add or remove instances based on demand. The WGU Cloud Deployment and Operations Study Guide (Section 3.2, Elastic Load Balancing) notes, "Elastic Load Balancing distributes traffic across EC2 instances, enabling horizontal scaling as part of an Auto Scaling group to handle increased load efficiently." Elastic IPs, elastic network interfaces, and Elastic Beanstalk do not provide this scaling functionality.

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The Reachability Analyzer in AWS VPC should be used to troubleshoot connectivity issues from on-premises computers to EC2 instances, especially when a network security group (NSG) is suspected. It analyzes the network path and identifies blocked traffic due to NSG rules or route tables. The WGU Cloud Deployment and Operations Study Guide (Section 3.2, VPC Troubleshooting) states, "Reachability Analyzer helps diagnose connectivity issues by simulating traffic paths, identifying if an NSG rule (e.g., blocked port) or route table is causing the problem from on-premises to EC2 public IPs." Flow Logs provide data but not analysis, PrivateLink is for private connections, and Traffic Mirroring is for monitoring.

The `disk_io_time` metric, collected by the CloudWatch agent on EC2 Linux instances, measures the total time (in milliseconds) that read and write requests have waited on I/O, helping to assess disk performance for instance sizing. The WGU Cloud Deployment and Operations Study Guide (Section 4.1, CloudWatch Agent Metrics) states, "The `disk_io_time` metric, available via the CloudWatch agent, tracks the time spent waiting for I/O operations, providing insight into disk contention for optimizing EC2 instance sizing." disk_read_ops, diskio_reads, and diskio_read_bytes measure operation counts or bytes, not wait times.

The `Ref` function in an AWS CloudFormation template is used to obtain values of components defined within the template, such as resource IDs, parameters, or outputs. This function allows dynamic referencing of resources during stack creation. The WGU Cloud Deployment and Operations Study Guide (Section 5.3, CloudFormation Functions) states, "The `Ref` function is used to reference the logical IDs of resources, parameters, or mappings defined in the template, enabling dynamic value retrieval (e.g., `Ref: MyEC2Instance` returns the instance ID)." ImportValue is used for cross-stack references, Cidr for IP range calculations, and GetAZs for availability zone lists, making them unsuitable here.