Mysterious metadata monster swamped Google’s blobs and crashed its cloud

Parts of the GCloud are more robust and survived, but the fix is not yet in for others

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Google has explained how and why big chunks of its cloud crashed last week, and as is often the case the company broke itself.

A Google Cloud Issue Summary [PDF] detailing the root cause of the outage starts by explaining “Many Google services use a common, internal, distributed system for immutable, unstructured data, also known as binary large objects, or blobs. This blob storage system contains a frontend which interfaces with Google-internal client services, a mid-layer which handles metadata operations, and backend storage for the blobs themselves. When clients make requests to the frontend, metadata operations are forwarded to the metadata service, which communicates with the storage service.”

Which is interesting inasmuch as it explains a little of Google’s operations, but doesn’t explain what went wrong.

The next paragraph gets into that, as follows:

An increase in traffic from another Google service started overloading the metadata service, causing tasks to become unhealthy and requests to increase in latency. This latency prompted excessive retries of these operations, leading to resource exhaustion.

While some tasks were able to start, many “were immediately overwhelmed by the amount of traffic they received, and tasks that did start were allocated insufficient resources due to exhaustion.”

And it all cascaded from there because “The issue was exacerbated by the strategies used to cancel and retry failed requests, which caused a multiplicative effect in traffic.”

The result? Up to six hours and 35 minutes of woe, depending on which Google service you use.

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But interestingly, no woe for users of Google Cloud Storage, because while that product relies on the same blob storage, its metadata service is isolated.

“The migration for GCS metadata isolation is ongoing for the ‘US’ multiregion, while all other migrations have been completed. As a result, impact to GCS customers was lessened, and this impact was limited to the ‘US’ multiregion,” Google’s team wrote. The report doesn’t explain which Google service caused the initial metadata mess that caused so much grief for other products. It has of course promised to fix all the things that went wrong and test new and more robust routines for handling this sort of mess in future.

The company has also offered an unusually verbose breakdown of exactly what the outage wrought, as follows:

  • Gmail: The Gmail service was unavailable for some users, and email delivery was delayed. About 0.73% of Gmail users (both consumer and G Suite) active within the preceding seven days experienced 3 or more availability errors during the outage period. G Suite customers accounted for 27% of affected Gmail users. Additionally, some users experienced errors when adding attachments to messages. Impact on Gmail was mitigated by 03:30, and all messages delayed by this incident have been delivered.
  • Drive: Some Google Drive users experienced errors and elevated latency. Approximately 1.5% of Drive users (both consumer and G Suite) active within the preceding 24 hours experienced 3 or more errors during the outage period.
  • Docs and Editors: Some Google Docs users experienced issues with image creation actions (for example, uploading an image, copying a document with an image, or using a template with images).
  • New Google Sites: Some users were unable to create new Sites, add new pages to Sites, or upload images to Sites. Additionally, there was almost a 100% error rate in creating Sites from a template during the incident period. Impact on Sites was mitigated by 03:00.
  • Chat: 2% of Google Chat users who tried sending messages experienced errors, and 16% of Chat users who attempted to forward a message to Gmail experienced errors.
  • Meet: Livestreams were fully down for the duration of the incident, and recordings were delayed due to impact on YouTube. Meet impact lasted from 21:00 to 01:15, and from 01:40 to 02:10.
  • Keep: Some Google Keep users were served 500 Internal Server Error responses or experienced delays with operations involving media.
  • Voice: The delivery of some outbound SMS messages with attachments failed. The delivery of some inbound voicemails, call recordings, and SMS was delayed. Impact on Voice was mitigated by 03:20. All voicemails and recordings have been delivered, with a maximum delay of 5.5 hours.
  • Jamboard: Some users experienced errors when attempting to upload images or copy documents containing images.
  • Admin Console: Some users experienced errors when uploading CSV files in the G Suite Admin Console. The error rate for these operations ranged between 15 and 40% during the outage period.
  • App Engine: App Engine Standard apps making calls to the Blobstore API saw elevated error rates. Peak error rates were below 5% in most regions, but peaked as high as 47% in us-west1 and 13% in us-central1. App Engine Standard apps making calls to the Images API saw error rates up to 66%.
    Inbound HTTP requests served by static files or Blobstore objects saw elevated errors, peaking at 1%.
    Deployment of apps that include static files failed with a message "The following errors occurred while copying files to App Engine: File https://storage.googleapis.com/.... failed with: Failed to save static file." Impact on App Engine was mitigated by 03:25.
  • Cloud Logging: Log messages written to Google Cloud Logging, including logs generated by Google, such as App Engine request logs, activity logs, and audit logs, were delayed up to 4 hours and 43 minutes. The backlog of logs was completely processed by 16:00. During the period of outage, API calls to write and read logs returned successfully, but reads returned incomplete results.
  • Cloud Storage: API calls to Google Cloud Storage buckets located in the "US" multiregion saw error rates up to 1%. Errors had entirely subsited [sic] by 00:31.

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