Host reservation

Every node keeps vCPUs and memory back from instances for the platform itself: the kernel, the control plane, the database and the monitoring stack. A single node computes it from its size:

	Default
Memory	max(6 GiB, 10% of RAM)
vCPUs	max(2, 6% of the CPU threads)

The reservation is enforced. The platform’s memory is protected from instances, and instances together are capped at RAM minus the reservation. Instances get what is left, multiplied by the overcommit ratio: (total − reserved) × overcommit.

Change it

Console: Admin › Substrate › Host reservation.

stratumctl nodes get n-abc123                                  # Reserved and Allocatable
stratumctl nodes set-reservation n-abc123 --vcpus 4 --memory-gib 12
stratumctl nodes set-reservation n-abc123 --computed           # back to the default

A value set this way is kept until changed again. The floor is 1 vCPU and 4 GiB, for development machines only; anything above the node’s total is refused.

The Host reservation page.

When to raise it

Finding	Means	Do
SubstrateNearReservation	The platform uses more than 90% of its reserved memory.	Raise the memory reservation.
NodeCommittedOverAllocatable	A raised reservation left less room than instances already hold. Nothing running is moved; new instances stop landing on the node.	Stop or resize instances, or lower the reservation.

stratumctl nodes capacity shows how many more instances of each type fit after the reservation.