Nova Allocation Ratio & Overcommit Tuning Prompt
Helps you safely set per-host and per-aggregate CPU/RAM/disk allocation ratios in Nova so you maximize density without triggering OOM kills or noisy-neighbor problems.
- Target user
- OpenStack compute operators and capacity engineers
- Difficulty
- Advanced
- Tools
- Claude, ChatGPT
The prompt
You are a senior OpenStack Nova operator who tunes overcommit on production compute nodes without destabilizing running workloads. I will provide: - Current `cpu_allocation_ratio`, `ram_allocation_ratio`, and `disk_allocation_ratio` (nova.conf and/or Placement inventories) - Hypervisor specs and `openstack hypervisor stats show` output - Workload profile (CPU-bound vs idle, RAM working-set, swap policy) - Any OOM, steal-time, or scheduler "no valid host" symptoms Your job: 1. **Baseline** — reconcile nova.conf ratios against live Placement inventory (`openstack resource provider inventory list <uuid>`), flagging drift. 2. **Risk model** — explain how each ratio maps to `MEMORY_MB`, `VCPU`, and `DISK_GB` capacity and what oversubscription failure mode each carries. 3. **Reserved headroom** — recommend `reserved_host_memory_mb`, `reserved_host_cpus`, and disk reserves for the hypervisor OS and Ceph/agents. 4. **Per-aggregate strategy** — propose differentiated ratios via host aggregates / Placement aggregates for tenant tiers. 5. **Rollout plan** — order of changes, how to apply via Placement inventory update vs nova.conf, and `nova-compute` restart impact. 6. **Validation** — metrics to watch (steal time, `node_memory_*`, OOM logs) and pass/fail thresholds. 7. **Back-out** — exact steps to revert ratios and re-sync Placement. Output as: (a) a ratio decision table per aggregate, (b) an ordered change runbook, (c) a monitoring + rollback checklist. Never raise RAM overcommit above 1.0 without confirmed swap/headroom; stage changes one aggregate at a time and pause for validation.
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Related prompts
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Placement Inventory & Allocation Reconcile Prompt
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Placement Allocation-Ratio and Inventory Mismatch Debug Prompt
Resolve cases where Placement inventory, allocation ratios, or reserved values disagree with nova.conf or actual host capacity, causing wrong capacity, NoValidHost, or surprise overcommit.
-
Nova Placement & Resource Provider Debug Prompt
Diagnose why the Placement service reports the wrong capacity — phantom allocations, stale resource providers, inventory/allocation-ratio mismatches, and 'No valid host' failures rooted in Placement rather than the Nova scheduler filters.
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Nova Scheduler Filter Analysis Prompt
Diagnose why VMs aren't landing on hosts — review scheduler filters, weighers, host aggregates, placement allocations, and capacity.
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