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2022-03-28: Cloud NAT Gateway port allocation approaching saturation

Incident DRI

EOC: @msmiley

Current Status

Autoscaling up the GKE node count to support pod autoscaling implicitly reserves NAT gateway ports. During today's peak workload, the gateway ran low on the pool of available ports to lease to hosts. This is a recurring trend, so we should add capacity soon to the NAT gateway's IP pool.

But the saturation metric was under-representing the actual capacity ceiling, so the alert triggers at a lower utilization percentage than we want.

Short-term corrective action is to fix the saturation metric: https://gitlab.com/gitlab-com/gl-infra/capacity-planning/-/issues/37

Longer-term corrective action is to add capacity, which will probably take the form of splitting the work among multiple NAT gateways with discrete IP pools and port reservation policies: https://gitlab.com/gitlab-com/gl-infra/capacity-planning/-/issues/35

Summary for CMOC notice / Exec summary:

  1. Customer Impact: No customer impact, as the gateway did not quite reach saturation.
  2. Service Impact: ServiceNAT
  3. Impact Duration: N/A
  4. Root cause: RootCauseSaturation - This is a Near Miss, as the gateway briefly reached 95% utilization.

Timeline

Recent Events (available internally only):

All times UTC.

2022-03-28

  • 15:24 - @msmiley declares incident in Slack.

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Edited by Matt Smiley