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chrisbirley's avatar
chrisbirley
Copper Contributor
Jan 21, 2025

ASHCI cluster with different RAM amounts

been looking through server requirements and things, and i cant see a definitive answer of whether i need to have the same amount of RAM per ASHCI host. i appreciate it may not be best practice due to failing over VMs within the cluster, and being in a position where you over provision RAM and could end up in a sticky situation, but given that you can not only set preferred owners, but also have the ability to set possible owners, you should be able to account for that. 

tldr: ASHCI cluster can i have have one node with 4TB of RAM and 5 nodes with 2TB of RAM

clustering
Hyper-V
  • kyazaferr's avatar
    kyazaferr
    Steel Contributor
    Jan 30, 2025
    • Azure Stack HCI does not require identical hardware across all nodes, including RAM.
    • Cluster-aware updating (CAU) and failover clustering work with mixed RAM configurations, but performance can vary.
    • Storage Spaces Direct (S2D) will work, but imbalances in RAM can lead to caching inefficiencies.

    2️⃣ Failover Considerations

    • If a VM with high RAM allocation fails over to a node with less RAM, it may fail to start due to insufficient memory.
    • Using VM Placement Policies (via SCVMM or PowerShell) allows you to set preferred and possible owners to avoid overload.
    • Memory overcommitment with dynamic memory is possible, but not ideal for production workloads.

    3️⃣ Performance Implications

    • Nodes with higher RAM can handle more VMs and better caching.
    • Nodes with lower RAM may experience bottlenecks when VMs are distributed unevenly.
    • The cluster balancer will attempt to distribute VMs, but manual tuning may be required.

    4️⃣ Best Practices if Using Mixed RAM

    ✅ Use Preferred & Possible Owners: Assign VMs based on available RAM.
    ✅ Set VM Startup Priorities: Ensure critical VMs start first if failover occurs.
    ✅ Monitor Memory Usage: Use Windows Admin Center or SCVMM to track cluster health.
    ✅ Test Failover Scenarios: Ensure VMs can be redistributed without major performance degradation.

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