Budget raw capacity for two-way and three-way mirrors

Source facts

Storage Spaces Direct mirroring places complete copies on different physical hardware. Two-way mirroring writes two copies and has 50 percent storage efficiency; Microsoft’s example needs at least 2 TB of physical capacity for 1 TB of data and two server fault domains. Three-way mirroring writes three copies, giving 33.3 percent efficiency and requiring at least three server fault domains. Microsoft recommends three-way mirroring when the cluster has more than two servers. Microsoft documentation.

Applicability

Use this review when comparing mirror layouts for a supported cluster. Distinguish raw drive capacity from the usable data target. Review the source’s failure-tolerance discussion and the complete design requirements before selecting a resiliency option.

DSE recommendation

Write a capacity worksheet that identifies the copy count, server count, and intended data volume. Ask the storage owner to review growth, maintenance, and recovery headroom separately from the basic copy calculation. Show the business owner both raw and usable quantities in the proposal. Keep performance and failure requirements alongside cost so the layout is not chosen from capacity efficiency alone.

Verification

Compare the implemented resiliency and reported capacity with the worksheet. Record the actual server and drive inventory, and investigate any unexplained difference in available space. Validate the workload through the approved storage acceptance process. Revisit the calculation when the data target or fault-domain design changes.

Official references

Microsoft Learn: Fault tolerance and storage efficiency on Azure Local and Windows Server clusters. Source reviewed September 8, 2026.