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1. IL-0366 Cloud Storage uploads and writes in us-central1 slowed and failed with HTTP 503 as traffic piled onto a few backends

   Google Cloud2026

   Symptoms

   From 04:00 US/Pacific on 8 October 2026, Google Cloud Storage upload and write operations in the us-central1 region became slow and failed. Clients mostly saw HTTP 503 UNAVAILABLE errors on WriteObject operations and client-side timeouts.

   High latencyHigh error rateLoad imbalance

   Trigger

   A configuration update to the Cloud Storage load balancing layer turned on a traffic sorting optimization that was meant to deal with local capacity constraints. The recurrence on 9 October came from an incomplete rollback of an upload planner feature.

   Root cause

   Configuration change

   Component

   Object storageDatabase

   Contributing factor

   No headroomOverlapping failures

   Detection

   Google engineering teams monitoring system telemetry detected the problem at 04:50 US/Pacific on 8 October, 50 minutes after it began.

   Remediation

   • Drained traffic from the most overloaded storage backend nodes
   • Deployed a configuration change that disabled the traffic sorting optimization
   • Rolled back a recent update to the metadata database service
   • Redirected traffic and drained affected storage components again during the recurrence
   • Fully reverted the load balancing configuration change, restoring capacity-aware routing
   • Returned the drained nodes to service once resource use was back to normal

   Action items

   • Validate load balancing logic changes in smaller regions before rolling them out to large, highly utilized regions
   • Make the storage load balancing enforce capacity constraints so traffic cannot bypass pre-spilling safeguards
   • Speed up internal data movement within highly utilized storage cells to absorb temporary load spikes

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