Introduction: In a VMware vSphere environment, APD (All Paths Down) and PDL (Permanent Device Loss) are storage-related conditions that can impact the availability and stability of virtual machines. Understanding these conditions and knowing how to troubleshoot them is crucial for maintaining a robust and reliable virtual infrastructure. In this comprehensive guide, we’ll explore the reasons for APD and PDL occurrences, and provide troubleshooting steps along with real-world scenarios to help you effectively handle these situations.

APD (All Paths Down): APD is a condition where the ESXi host loses all communication paths to a storage device. This can happen due to various reasons, such as a temporary storage outage, storage controller failure, or network connectivity issues. When APD occurs, the ESXi host cannot reach the storage device, leading to potential I/O failures and temporary unavailability of virtual machines.

Causes of APD:

1. Storage Outage: A temporary loss of connectivity between the ESXi host and the storage device due to a network disruption or maintenance activity.
2. Storage Controller Failure: The storage controller experiences a hardware or software failure, resulting in the loss of all paths to the storage device.
3. Firmware or Driver Issues: Incompatibility or bugs in storage controller firmware or driver versions can lead to APD conditions.
4. Resource Contentions: Resource contentions, such as CPU or memory issues, may affect the ESXi host’s ability to maintain storage paths.

Troubleshooting APD:

1. Monitoring and Alerts: Use vCenter alarms and notifications to detect APD conditions early and take proactive action.
2. Log Analysis: Review ESXi host logs (e.g., vmkernel.log) and storage array logs for any APD-related entries.
3. Investigate Storage Infrastructure: Check the storage device, storage network, and storage controller for errors or hardware issues.
4. Firmware and Driver Updates: Ensure storage controller firmware and driver versions are up to date and compatible with ESXi.
5. VMware HCL: Validate that the storage hardware is listed on the VMware Hardware Compatibility List (HCL) to ensure compatibility.
6. Adjust Timeout Values: Fine-tune APD-related timeout settings (e.g., Disk.AutoremoveOnPDL) based on your specific requirements.

APD Scenario:

Scenario: A network maintenance activity accidentally causes a temporary network disruption between an ESXi host and its storage device. As a result, the ESXi host loses all communication paths to the storage, triggering an APD condition.

Troubleshooting Steps:

1. Check vCenter Alarms: Monitor vCenter alarms to detect the APD condition and receive alerts.
2. Review ESXi Host Logs: Analyze the vmkernel.log on the affected ESXi host to identify any APD-related entries.
3. Validate Network Connectivity: Verify that the network connection between the ESXi host and the storage device has been restored.
4. Check Storage Array Status: Investigate the storage array logs and management interface for any indications of connectivity issues.
5. Rescan Storage Adapters: Perform a storage adapter rescan on the ESXi host to re-establish connectivity with the storage device.
6. Monitor VM Behavior: Monitor virtual machine behavior to ensure they resume normal operations after the APD condition is resolved.

PDL (Permanent Device Loss): PDL is a condition where the ESXi host acknowledges that a storage device has been permanently removed or lost. This can occur when a storage device fails, is disconnected, or is decommissioned. Once PDL is detected, the ESXi host marks the storage device as permanently inaccessible and takes necessary actions to avoid potential data corruption.

Causes of PDL:

1. Storage Device Failure: A permanent hardware failure in the storage device leads to PDL detection.
2. Storage Decommissioning: The storage device is intentionally removed from the environment, leading to PDL.
3. Storage Device Disconnection: The storage device is accidentally disconnected from the ESXi host, causing PDL.

Troubleshooting PDL:

1. Log Analysis: Review ESXi host logs (e.g., vmkernel.log) to identify any PDL-related entries.
2. Validate Storage Device: Confirm the status of the storage device and verify if it has been intentionally removed or failed.
3. Check Connectivity: Ensure that the storage device is correctly connected and accessible by the ESXi host.
4. Remove Dead Paths: Manually remove any dead paths to the storage device using the esxcli command-line interface.
5. Check Multipathing Configuration: Review and adjust multipathing settings to ensure proper handling of PDL conditions.

PDL Scenario:

Scenario: A storage controller failure leads to the permanent loss of connectivity between an ESXi host and its storage device. The ESXi host detects the PDL condition as it acknowledges the permanent loss of the device.

Troubleshooting Steps:

1. Analyze vmkernel.log: Review the vmkernel.log on the affected ESXi host to detect PDL entries.
2. Check Storage Device Status: Confirm the status of the storage device and verify if it has indeed failed or been decommissioned.
3. Verify Storage Controller Health: Investigate the storage controller for any hardware or software failures.
4. Remove Dead Paths: Manually remove any dead paths to the affected storage device using the esxcli command-line interface.
5. Validate Multipathing Configuration: Ensure that the multipathing configuration is appropriately set to handle PDL conditions.

Conclusion: APD and PDL are critical storage-related conditions that can impact the availability and stability of virtual machines in a VMware vSphere environment. By understanding the causes and troubleshooting steps outlined in this guide, you can effectively address APD and PDL situations and ensure the continued reliability and performance of your virtual infrastructure. Remember to use VMware’s official documentation, support resources, and best practices while troubleshooting and handling these conditions. Always test any changes or solutions in a non-production environment before implementing them in a production setting.