OpenStack Nova 'No Valid Host Was Found': A Scheduling Failure Triage Guide
Why Nova reports 'No valid host was found': triage compute services, Placement inventory drift, scheduler filter rejections, cell mapping, and image reachability.
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openstackNo valid host was found in Nova Scheduler after DevStack cycle
DevStack is documented as a development environment rather than a reproducible deployment, and consecutive stack.sh runs can diverge when residual state is left behind. One documented failure condition that can surface in this situation is the Nova Scheduler message No valid host was found , raised when no compute host matches placement, resource, and filter
Nova API ↔ Placement Service: Does Shared Database Connection Pool Cause Latency Spikes Under Concurrent Requests?
Goal: Determine whether latency spikes observed under concurrent Nova API requests stem from contention in the shared oslo.db connection pool used by both Nova API and Placement service, and whether adjusting the database max_pool_size relative to the total number of API workers eliminates the spike. Constraints: The effect is documented for OpenStack releas
Placement-backed scheduling in Nova: attributing latency after the Stein and Train transitions
Nova's scheduler has moved from host-side evaluation to Placement-backed candidate selection. Before Stein, the FilterScheduler applied filters and weighers to compute-node state it gathered itself. From Stein (19.0.0) onward, the scheduler requests allocation candidates from the Placement service, which returns a pre-filtered set, and Train (20.0.0) shifted
Nomad Placement: Binpack Density vs Spread Availability Under a Fixed Node Budget
A Nomad job specification can express placement preference per task group with spread and binpack blocks. Under a fixed node budget, the choice is a trade-off rather than a ranking: binpack concentrates allocations onto fewer clients, reducing idle capacity, while spread distributes allocations across eligible nodes, limiting the blast radius of a single nod