Cases12 cases

7 to 11 of 12

1. IL-0019 An EKS Terraform module upgrade shrank node root disks from 100 GB to 20 GB and image pulls failed with no space left on device

   UK Ministry of Justice2023

   Symptoms

   At 13:42 the RootVolUtilisation-Critical alert fired in the high-priority alerts channel. At 14:03 a user reported image pull errors with the message "no space left on device".

   Node disk fullImage pull failuresPod evictionPods stuck in ContainerCreatingIP address exhaustion

   Trigger

   Upgrading the EKS Terraform module to version 18, which changed the node launch template, and draining workloads onto the new nodes it created.

   Root cause

   Configuration change

   Component

   Node root diskNode launch templateNode poolAWS VPC CNICoreDNS

   Contributing factor

   Missed in reviewNo configuration testsNo alert on limit

   Detection

   The RootVolUtilisation-Critical alert fired in the high-priority alerts channel at 13:42, and a user reported "no space left on device" image pull errors at 14:03.

   Remediation

   • Uncordoned the old default node group and drained the new nodes to move workloads back
   • Identified the launch template change that cut the root volume to 20 GB and applied a fix on the manager and live clusters
   • Drained the old node group onto the fixed nodes

   Action items

   • Update the runbook to compare launch template changes during an EKS module upgrade
   • Create a test setup that pulls images of different sizes, like live
   • Update the RootVolUtilisation alert runbook to check the disk space configuration
   • Scale CoreDNS dynamically based on the number of nodes
   • Investigate whether IPv6 can solve the IP prefix starvation problem
   • Add drift testing to identify when a terraform plan shows a change to the launch template
   • Set up logging for the CNI and ipamd logs and alerts on errors related to IP prefix starvation

   Lessons

   The write-up lists review actions rather than lessons. They aim at two gaps: the launch template changes brought by a module upgrade were not compared or drift-tested before rollout, and there were no CNI or ipamd logs or alerts to show the IP prefix starvation the team suspected during the drain.

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2. IL-0043 Application logs missing in Kibana after a newer Fluent Bit version changed its chunk drop strategy

   UK Ministry of Justice2023

   Symptoms

   At 09:14 on 4 August 2023, users of the Cloud Platform reported in the #ask-cloud-platform Slack channel that Kibana was missing their application logs for long periods.

   Monitoring outage

   Trigger

   The newer Fluent Bit version in use had changed its chunk drop strategy; the fix that followed handled memory buffer overflow.

   Root cause

   Capacity exhaustion

   Component

   Telemetry agent

   Contributing factor

   No alert on limitMonitoring blind spot

   Detection

   Users reported long periods of missing logs in Kibana in the #ask-cloud-platform Slack channel at 09:14; the incident was declared at 10:09.

   Remediation

   • configured Fluent Bit to handle memory buffer overflow by writing buffered logs to the file system
   • made Fluent Bit flush logs in smaller chunks

   Action items

   • Push notifications from the logging clusters to #lower-priority-alerts (#4704)
   • Add an integration test that checks logs are being sent to the logging cluster

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3. IL-0016 Prometheus on the live cluster ran out of memory and was OOMKilled, and the monitoring node it ran on went NotReady

   UK Ministry of Justice2023

   Symptoms

   At 14:05 on 25 July 2023, a high-priority PagerDuty alert from Pingdom reported that the "Prometheus - live healthcheck" was DOWN. Prometheus on the live cluster was unavailable, and the Cloud Platform lost its monitoring for a while.

   Monitoring outageOOMKilledNode NotReady

   Trigger

   The growing number of namespaces and Prometheus rules raised Prometheus' memory needs beyond what its node instance size could hold.

   Root cause

   Capacity exhaustion

   Component

   PrometheusNode poolContainer memory limit

   Contributing factor

   No alert on limit

   Detection

   A Pingdom health check on Prometheus failed and raised a high-priority PagerDuty alert.

   Remediation

   • drained the monitoring node, moving Prometheus to another monitoring node
   • changed the node instance size to r6i.4xlarge
   • raised the Prometheus memory limit to 100Gi
   • set the Prometheus container limits to 12 CPU cores and 110Gi of memory

   Action items

   • Add an alert on node memory usage and on a pod using up most of a node's memory (issue #4538)

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4. IL-0184 JupyterHub's KubeSpawner could not create containers and kubectl was refused because the API server and other internal certificates of a kubeadm cluster had expired

   dbhang2021

   Symptoms

   KubeSpawner, which starts JupyterHub user containers on Kubernetes, failed to create containers with an error the author had never seen, saying an SSL certificate had expired.

   Control plane unresponsiveDeploys failing

   Root cause

   Certificate or credential expiry

   Component

   Kubernetes API serverkubelet

   Contributing factor

   Unknown limitNo alert on limit

   Detection

   KubeSpawner failed to create a container and logged an SSL certificate verification error.

   Remediation

   • checked certificate expiry with kubeadm alpha certs check-expiration
   • moved the old certificates and /etc/kubernetes/*.conf files to a backup directory
   • regenerated all certificates with kubeadm init phase certs all
   • regenerated the kubeconfig files with kubeadm init phase kubeconfig all
   • copied /etc/kubernetes/admin.conf to ~/.kube/config and restarted kubelet
   • restarted the kube-system containers with docker restart so they picked up the new credentials

   Lessons

   kubeadm clusters run on certificates that expire, and when the API server certificate expires kubectl cannot be used to fix it, so renewal has to happen on the control plane node with kubeadm. After renewal, the kubeconfig files and the running control plane containers must also pick up the new certificates, or kubectl returns Unauthorized.

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5. IL-0013 The conntrack table filled up on downsized GKE nodes and a high-traffic aggregation proxy saw climbing connection resets

   loveholidays2020

   Symptoms

   Shortly after a round of changes, loveholidays saw connectivity problems and raised error rates on its aggregation service. The service is a proxy between two other services of more than 100 instances each, handles over 10,000 requests per second continuously and can spike to four times that.

   Connection resetsHigh error rateIntermittent connection failures

   Trigger

   Three changes together: a change in service architecture, halving the RAM of the nodes that run the aggregation service, and sharding a downstream service, which doubled the number of connections to track.

   Root cause

   Capacity exhaustion

   Component

   conntrack tablekube-proxyNode poolHAProxy

   Contributing factor

   Unknown limitNo capacity planningNo alert on limit

   Detection

   Rising error rates and climbing connection reset errors in HAProxy, then node_exporter conntrack metrics compared with their limit.

   Remediation

   • moved the aggregation service to nodes with more RAM
   • limited the number of aggregation service pods per node

   Action items

   • Alert when conntrack table use goes above 75%, with (node_nf_conntrack_entries / on (pod) node_nf_conntrack_entries_limit / on (pod) group_right kube_pod_info) > 0.75

   Lessons

   The conntrack limit is tied to node memory, so resizing a node pool silently resizes the connection table, and on GKE the ratio did not match the commonly cited formula. A high-traffic proxy on small nodes can exhaust it, and the symptoms are connection resets and timeouts, so conntrack use needs its own metric and alert.

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