ALUA stands for Asymmetric Logical Unit Access. It is a feature in storage area networks (SANs) that allows for more efficient and optimized access to storage devices by different paths, particularly in environments with active/passive storage controllers.

In traditional active/passive storage arrays, one controller (path) is active and handling I/O operations while the other is passive and serves as a backup. ALUA enhances this setup by allowing hosts to intelligently direct I/O operations to the most appropriate and optimized path based on the state of the storage controllers.

Here’s why ALUA is used and its benefits:

1. Optimized I/O Path Selection: ALUA-enabled storage arrays provide information to the host about the active and passive paths to a storage device. This enables the host to direct I/O operations to the active paths, reducing latency and improving performance.
2. Load Balancing: ALUA helps distribute I/O traffic more evenly across available paths, preventing congestion on a single path and improving overall system performance.
3. Improved Path Failover: In the event of a path failure, ALUA-aware hosts can quickly switch to an available active path, reducing downtime and maintaining continuous access to storage resources.
4. Enhanced Storage Controller Utilization: ALUA allows hosts to utilize both active and passive paths for I/O operations, maximizing the usage of available resources and ensuring better storage controller utilization.
5. Reduced Latency: By directing I/O operations to active paths, ALUA reduces the distance data needs to travel within the storage array, resulting in lower latency and improved response times.
6. Better Integration with Virtualization: ALUA is particularly beneficial in virtualized environments where multiple hosts share access to the same storage resources. It helps prevent storage contention and optimizes I/O paths for virtual machines.
7. Vendor Compatibility: ALUA is widely supported by many storage array vendors, making it a standardized approach for optimizing I/O operations in SAN environments.

ALUA configuration involves interactions between the ESXi host, storage array, and vCenter Server, and the process can vary depending on the storage hardware and vSphere version you are using.

When configuring the Path Selection Policy (PSP) for Asymmetric Logical Unit Access (ALUA) in a VMware vSphere environment, the best choice of PSP can depend on various factors, including your storage array, workload characteristics, and performance requirements. Different storage array vendors may recommend specific PSP settings for optimal performance and compatibility. Here are a few commonly used PSP options for ALUA:

1. Round Robin (RR):
   • PSP: Round Robin
   • IOPS Limit: Set an appropriate IOPS limit per path to control path utilization.
   • Use Case: Round Robin with an IOPS limit can help distribute I/O across available paths while still adhering to the ALUA principles. It provides load balancing and redundancy.
2. Most Recently Used (MRU):
   • PSP: Most Recently Used (MRU)
   • Use Case: In some cases, using MRU might be suitable when the storage array already optimizes path selection based on its own logic.
3. Fixed (VMW_PSP_FIXED):
   • PSP: Fixed (VMW_PSP_FIXED)
   • Use Case: Some storage arrays require using the Fixed PSP to ensure optimal performance with their ALUA implementation. Consult your storage array vendor’s recommendations.

It’s important to note that the effectiveness of a PSP for ALUA depends on how well the storage array and the ESXi host work together. Some storage arrays might have specific best practices or recommendations for configuring PSP in an ALUA environment. It’s advisable to consult the documentation and guidance provided by your storage array vendor.

Configuring Asymmetric Logical Unit Access (ALUA) and Path Selection Policies (PSPs) in a VMware vSphere environment involves using the vSphere Client to select and configure the appropriate PSP for storage devices that support ALUA. Here’s a step-by-step guide with examples:

1. Log into vCenter Server: Log in to the vSphere Client using your credentials.
2. Navigate to Storage Adapters:
   • Select the ESXi host from the inventory.
   • Go to the “Configure” tab.
   • Under “Hardware,” select “Storage Adapters.”
3. View and Configure Path Policies:
   • Select the storage adapter for which you want to configure ALUA and PSP.
   • In the “Details” pane, you will see a list of paths to storage devices.
   • To configure a specific PSP, you’ll need to adjust the “Path Selection Policy” for the storage device.
4. Configure Path Selection Policy for ALUA:
   • Right-click on the storage device for which you want to configure ALUA and PSP.
   • Select “Manage Paths.”
5. Choose a PSP for ALUA:
   • From the “Path Selection Policy” drop-down menu, select a PSP that is recommended for use with ALUA. Examples include:
     • “Round Robin (VMware)” with an IOPS limit.
     • “VMW_PSP_ALUA” (if available and recommended by the storage vendor).
6. Adjust PSP Settings (Optional):
   • Depending on the selected PSP, you might need to adjust additional settings, such as IOPS limits or other parameters. Follow the documentation provided by your storage array vendor for guidance on specific settings.
7. Monitor and Verify:
   • After making changes, monitor the paths and their states to ensure that the chosen PSP is optimizing path selection and load balancing effectively.
8. Repeat for Other Devices:
   • Repeat the above steps for other storage devices that support ALUA and need to be configured with the appropriate PSP.
9. Test and Optimize:
   • In a non-production environment, test the configuration to ensure that the chosen PSP and ALUA settings provide the expected performance and behavior for your workloads.