Cases2 cases

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1. IL-0133 A storm shut down cooling at Nebius's us-central1 data center, and a disk hot-plug bug kept Managed Kubernetes control planes down for hours after power returned

   Nebius2026

   Symptoms

   On 19 August 2026 the first equipment-level signs in Nebius's us-central1 region appeared around 09:40 UTC: the InfiniBand fabric degraded and GPU over-temperature alerts began to accumulate. Customer reports followed shortly after 10:00. At 10:15 on-call engineers were paged on critical component temperature alerts, and the incident was declared at 10:21.

   Control plane unresponsiveService outageMonitoring outage

   Trigger

   A storm-related event at the data center building took the building management system offline and shut down the chilled-water cooling loop.

   Root cause

   Dependency failure

   Component

   Data center power and coolingCloud block storageetcdInfiniBand fabric

   Contributing factor

   Single point of failureMonitoring blind spotNo alert on limitNo runbookOverlapping failures

   Detection

   No facility-level alert fired, because the storm also disabled the building management system that monitors the facility. Engineers acted on component over-temperature alerts that paged on-call at 10:15 UTC, after InfiniBand degradation, GPU over-temperature alerts and customer reports; the temperature rise from about 09:15 was visible on dashboards but had no alert.

   Remediation

   • had the facility operator switch cooling to manual bypass mode and opened rack rear doors to vent heat
   • re-energised tripped racks, control-plane racks first
   • restored the regional infrastructure control plane by manual intervention
   • restarted block-storage host components to clear stale disk attachment state
   • raised the VM recovery rate limits by hand for the region-scale recovery
   • started the affected Managed Kubernetes control-plane instances by hand
   • applied a controlled stop and start to control-plane instances whose data disk had been left unattached
   • identified and started 104 customer VMs that the incident had stopped

   Action items

   • Complete a joint resilience review with the facility operator covering redundancy of facility-management and monitoring systems, cooling that keeps running when those fail, electrical protection of cooling equipment and on-site response
   • Add facility-level environmental alerting on cooling plant state and room temperature trend that pages on-call independently of the building management system
   • Set up a documented emergency escalation path with the facility operator, with an on-call contact tree and guaranteed response times
   • Define an emergency heat-load reduction procedure with criteria, ownership and mechanics for shutting down or degrading workloads during a cooling loss
   • Establish a dependency-ordered region cold-start procedure with clear ownership for each service layer
   • Run regular region-scale recovery drills that cover VMs, storage attachments and Managed Kubernetes clusters
   • Fix the disk-attachment bug that extended the Managed Kubernetes outage and deploy the fix to all production regions
   • Improve the recovery policies Compute users can configure, and move all Managed Kubernetes control-plane nodes to the improved policy
   • Automate reconciliation of stale storage attachment state after abrupt host failures
   • Remove in-region dependencies from monitoring startup and recovery, and extend cross-region monitoring
   • Make regional health checks used for global traffic management reflect real service availability

   Lessons

   Facility monitoring that runs on the same system it watches goes silent in exactly the failure it should report, and with no alert on the room temperature trend there was no practical window to shed heat load in a controlled way. A cold start of a whole region exercised recovery paths built for isolated failures: single-attempt VM recovery, conservative rate limits and stale disk attachments all needed manual work. The longest part of the outage was restoring customer resources, Managed Kubernetes control planes above all, so Nebius is extending its recovery drills to that layer.

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2. IL-0135 A power source switch during UPS maintenance cut power to a row of GPU servers and shut down Crusoe Cloud VMs in us-northcentral1

   Crusoe Cloud2023

   Symptoms

   At 20:31 UTC on 15 November 2023, some GPU virtual machines in us-northcentral1-a became unavailable. One row of servers in a Crusoe Cloud data center in the us-northcentral1 region had lost power, and every server in the row powered off together with the VMs running on it.

   GPU unavailableService outage

   Trigger

   While the row's UPS was in bypass mode for routine maintenance, power switched from the primary source to a backup source for reasons unrelated to the maintenance.

   Root cause

   Dependency failure

   Component

   Data center power and cooling

   Contributing factor

   Single point of failureOverlapping failures

   Detection

   The power failure happened at 20:31 UTC. The first status post, at 21:07 UTC, said certain VMs in us-northcentral1-a were unavailable, and at 21:31 UTC the status page said the affected VMs had been identified.

   Remediation

   • identified the VMs on the row that lost power
   • restarted the affected VMs after power was restored

   Action items

   • Review the standard operating procedures for UPS maintenance to minimise customer impact during future maintenance
   • Build future data centers with redundant UPSs
   • Improve recovery procedures to reduce mean time to resolution

   Lessons

   When the only UPS for a row is in bypass for maintenance, the row is exposed to any power event during that window, even one unrelated to the work. Crusoe Cloud's response is redundant UPSs in future data centers, a review of maintenance procedures, and faster recovery of VMs after power returns.

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