At 2019-10-21 01:38 UTC the docs.rs website went down because no available disk space was left on the server hosting the application. Crate builds were failing since 2019-10-20 00:55 UTC due to the same reason.

Root cause of the outage

docs.rs needs to store the built documentation on the filesystem before uploading it to the database, and it does so in the /opt/docs-rs-prefix/documentations directory. docs.rs never cleared that directory though, so over time it started to increase its size until it caused this outage. Code to periodically purge temporary directories was present, but it was never configured to purge the one which caused the outage.

Resolution

As the directory doesn’t contain any persistent data we cleared it and the web server was restarted. Once we were confident the situation was resolved all the crates that failed due to the outage were queued for a rebuild.

Postmortem

The increased disk usage was gradual over weeks, slowly reaching 100% and causing the outage. While monitoring systems were in place and recorded graphs of the increase, no alert was configured so nobody noticed the problem. We need to add alerts when disk usage reaches 90%, so the problem can be investigated and dealt with on time.

Crates started to fail to build a day earlier, and close to no builds were successfully completed since then. We need to setup alerts when most of the builds are failing: as we don’t have the necessary metrics at the moment to reliably alert we'll have to add extra instrumentation as well.

Our response was slower due to issues with our on-call rotation for the service. The primary contacts don’t have the level of access required to increase the disk space of the instance (the temporary fix that was investigated at first but discarded after the discovery nobody awake could do it), and the backup contacts don’t have any production access or expertise on docs.rs.

Timeline of events

Unless otherwise noted all events happened on 2019-10-21, and all times are in UTC.

• 2019-10-20 00:55: crate builds started failing due to the low disk space
• 01:38: alerts fired for the docs.rs website being down, ashleygwilliams (backup contact) got paged
• 01:39: QuietMisdreavus joins into the operations channel
• 01:39: QuietMisdreavus found the reason for the outage (full root partition)
• 01:52: ashleygwilliams proposed to increase disk space, nobody with permissions required to so was awake or available though
• 01:56: ashleygwilliams contacts Mark-Simulacrum, who has the access required to increase disk space
• 01:57: QuietMisdreavus found the directory taking up all the disk space
• 02:00: QuietMisdreavus removed the directory taking up all the disk space
• 02:03: QuietMisdreavus restarted the web server
• 02:06: CDN propagated the changes, docs.rs back online
• 02:06: Mark-Simulacrum joins into the operations channel
• 08:19: pietroalbini added builds failed during the outage back into the queue
• 19:27: builds of the crates failed during the outage finished

Action items

• Update the docs.rs source code to cleanup the offending directory automatically.
• Add alerts when the available disk space on a server is below 10%.
• Add alerts when most of the builds are failing.
• Revisit the on-call rotation to make sure everyone on it has the permissions to either react to the incidents or escalate.