Revisit mirror policy when a Storage Spaces Direct cluster grows

Source facts

Adding a server expands storage capacity and can also add compute resources in a hyperconverged deployment. Microsoft describes two-way mirroring for two-server clusters and the availability of three-way mirroring when a third server is added. The mirror resiliency setting can be changed so newly created mirrored volumes default to three-way mirroring. An existing two-way mirror cannot become a three-way mirror through an in-place upgrade; Microsoft instead describes copying data to a new volume. Microsoft documentation.

Applicability

Use this review when an existing Storage Spaces Direct cluster expands from two servers. Identify the existing volume layouts and the intended policy for new volumes. Distinguish adding hardware from deciding how subsequent storage will use it.

DSE recommendation

Have the storage owner review the new resiliency choice alongside capacity and recovery requirements. Record which volumes are existing and which will be created after expansion. Approve any default-setting change explicitly and document its effect on future provisioning. Plan any existing-data migration separately, including new-volume capacity and verification before authorizing removal of an old volume.

Verification

Inspect cluster membership and health after the approved addition. Create a controlled test volume through the intended provisioning workflow and inspect its actual resiliency. Compare the result with the new-volume policy and capacity plan. Retain existing-volume observations separately so an unchanged layout is not mistaken for a failed default-setting change.

Official references

Microsoft Learn: Adding servers or drives to Storage Spaces Direct. Source reviewed September 8, 2026.