How to Calculate Actual Usable Space in aSAN (2-Copy Policy)
  

George Fady Lv2Posted 2026-Jun-24 21:31

One of the most common mistakes in new deployments is assuming that Raw Storage = Usable Space. In Sangfor aSAN, the math is different because the system prioritizes reliability over raw volume.
The Engineering Standard: When using a 2-replica (2-Copy) policy, the formula to determine real-world capacity is: Available Storage Capacity = (Total Raw Data Disks / 2) × 85%.
Why the 85%? This 15% reduction accounts for:
  • System File Reservations: Space held for the HCI firmware and log partitions.
  • Metadata Overhead: The distributed file system needs space to track data block locations across nodes.
  • Stability Margin: Ensuring the storage pool doesn't hit 100%, which would cause I/O freezes.
Real-World Example: If you have 24TB of raw HDD, your actual usable space for VMs is roughly 10.2TB.

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