Incident Library
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Cases2 cases

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  1. IL-0367 Azure Storage in Central US rejected VM disk I/O, taking down VMs and the SQL Database and Cosmos DB services on them
    Microsoft Azure2024
    Symptoms

    From 21:40 UTC on 18 July 2024, virtual machines in the Azure Central US region lost access to their disks, and the services running on those VMs failed. Azure SQL Database and SQL Managed Instance showed unhealthy nodes and login failures, Cosmos DB requests failed, and service management operations failed across many Azure services.

    Storage disconnected or stale mountsService outageHigh error rateFailover failureStale endpoint routing
    Trigger

    Routine changes to the VM host fleet led to a new allow list of VM host address ranges being generated and published to all Storage scale units in Central US. Recent network configuration changes had made some of the infrastructure file servers that hold the source data unreachable from the server generating the list, so the published list was incomplete.

    Root cause
    Configuration change
    Component
    Cloud block storageDatabaseDatabase failover manager
    Contributing factor
    Unsafe failure behaviourNo staged rolloutExcessive blast radiusMonitoring blind spot
    Detection

    Service monitoring detected the drop in VM availability at 22:06 UTC while the allow list update was being published. SQL monitoring detected unhealthy nodes and login failures within one minute of SQL becoming unavailable, and monitoring flagged the availability drop for Cosmos DB, SQL DB and SQL DB Managed Instance at 22:14 UTC.

    Remediation
    • Halted the allow list update workflow worldwide
    • Paused all deployments in the Central US region
    • Paused compute buildout deployments in all regions
    • Updated the configuration of all Storage scale units in Central US, restoring storage availability by 02:55 UTC
    • Started geo-failovers for SQL databases with automatic failover policies
    • Restarted SQL gateway nodes to refresh their caches and route connections to the right nodes
    • Forced completion of SQL failovers that had not completed
    • Failed over the Cosmos DB control plane and active-passive accounts with a write region in Central US
    • Cancelled customer-triggered graceful switch region operations that blocked Cosmos DB failovers
    • Removed locks on Cosmos DB service metadata to force-complete stuck failovers
    • Repaired impacted Cosmos DB partitions for single-region accounts
    • Repaired backend nodes and failed Cosmos DB accounts back to Central US, redoing failback where it failed
    Action items
    • Make the allow list generation workflow detect incomplete source information and halt
    • Alert on storage requests rejected by allow list checks
    • Deploy allow lists one Availability Zone and storage type at a time, over 24 hours
    • Add VM health checks and an automatic stop to the allow list deployment workflow
    • Reevaluate the policy for Microsoft-managed failover of SQL failover groups
    • Fix the Cosmos DB failback workflow that broke some MongoDB drivers for MongoDB API accounts
    • Fix Cosmos DB failback for accounts with private endpoints so it does not need redoing
    • Make the storage data-plane firewall reject invalid allow lists and keep the last-known-good state
    • Improve Azure NetApp Files monitors so impacting events are detected and classified in time
    • Monitor more Azure NetApp Files service metrics to correlate similar events faster
    • Make the SQL Service Fabric cluster location change notifications reliable under load
    • Make SQL geo-failover workflows more robust so they complete
    • Remove the issues that delayed 5% of Cosmos DB failovers
    • Migrate Azure DevOps customer metadata to the right geography
    • Decouple Azure NetApp Files regional reads and writes to keep this class of issue to one region
    • Evaluate caching to reduce Azure NetApp Files reliance on stored utilization data
    • Add automatic per-partition failover for Cosmos DB multi-region active-passive accounts
    • Build the Resilient Ephemeral OS disk improvement so VMs survive storage incidents better
    View case
  2. IL-0461 Every rollout of Datadog's metrics query service on Kubernetes raised errors that the logs blamed on DNS
    Datadog2022
    Symptoms

    From September 2021, each rollout of an update to Datadog's metrics query service, which serves real-time and historical metric data to the web frontend and to monitor evaluation, came with a rise in errors. Clients retried failed queries, so few errors reached users, but the retries slowed dashboards and monitors. The service's logs showed DNS errors stopping it from connecting to the data stores it depends on.

    High error rateDNS resolution failuresHigh latencyOOMKilledPacket lossStale endpoint routing
    Trigger

    Rollouts of the metrics query service, which delete the old pods. The client behaviour behind the failures came from an August 2021 rollout of the metrics query clients that applied round_robin as the new default gRPC load balancing policy.

    Root cause
    Configuration change
    Component
    conntrack tableCiliumLinux kernelCoreDNSClient-side DNS resolver and cacheexternal-dnsHTTP and RPC client library
    Contributing factor
    Change with a delayed effectRetry stormUnknown limitMonitoring blind spot
    Detection

    Errors rose during rollouts of the metrics query service, and the service's logs showed DNS errors when connecting to its data stores.

    Remediation
    • Raised the memory of the node-local-dns pods from 64 MB to 256 MB, which stopped the OOM kills but not the DNS errors
    • Tested larger instance types and confirmed that scaling up the service's infrastructure resolved the errors
    • Removed the custom gRPC reconnect parameters from the clients to restore the defaults
    Action items
    • Contribute a change to Cilium that adds an unreachable route for a deleted pod's IP so clients get an ICMP error
    Lessons

    The errors looked like DNS but were not. ENA metrics and VPC Flow Logs gave the visibility into VPC connection tracking that host metrics did not, and Cilium, Kubernetes and AWS networking interact in ways that can make reverse path filtering drop packets it should allow. DNS propagation time depends on many settings. The causes were changes Datadog had made itself, to work around a dependency bug, to resolve earlier incidents and to override default reconnect parameters, which looked safe at the time and showed their effects much later, when the link was hard to see.

    View case
Check your cluster for this

These 2 cases involve Cloud block storage and Cilium. kubegap audits a Kubernetes cluster for gaps like these, read-only, from Claude Code or Codex; its checks cite public reports like the ones in this library.

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By root cause1

Configuration change2 cases

Component
Cloud block storage1Cilium1conntrack table1CoreDNS1Database1Client-side DNS resolver and cache1external-dns1Database failover manager1HTTP and RPC client library1Linux kernel1
Contributing factor
Monitoring blind spot2Excessive blast radius1Change with a delayed effect1Unknown limit1No staged rollout1Retry storm1Unsafe failure behaviour1
Organisation
Datadog1Microsoft Azure1
  1. IL-0367 Azure Storage in Central US rejected VM disk I/O, taking down VMs and the SQL Database and Cosmos DB services on them
    Microsoft Azure2024
  2. IL-0461 Every rollout of Datadog's metrics query service on Kubernetes raised errors that the logs blamed on DNS
    Datadog2022