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At approx 2014-09-19 08:50:00 enwiki experienced a site outage. The apparent order of events was:

1. Three edits made to a template:

2. Jobrunner write activity[1] from wikiadmin user on enwiki master increased substantially, with cirrus in the spotlight:

Binlog showed substantial LinksUpdate hits (it is often in the top 10, but blends in with similar numbers to other traffic):

905809 LinksUpdate::incrTableUpdate
359301 LinksUpdate::updateLinksTimestamp
  9372 Title::invalidateCache
  8728 FlaggableWikiPage::clearStableVersion
  8099 User::invalidateCache
  5753 Revision::insertOn
  4692 ArticleCompileProcessor::save
  4607 `heartbeat`.`heartbeat`
  4404 FlaggedRevs::clearStableOnlyDeps
  3261 CheckUserHooks::updateCheckUserData
  3257 SiteStatsUpdate::tryDBUpdateInternal
  3185 RecentChange::save

The job activity occurred in waves with periods of very heavy writes, then minutes of nothing.

3. enwiki slaves started to experience intermittent replication lag. The main offender was:

DELETE /* LinksUpdate::incrTableUpdate

4. Surges of wikiuser DB connections to slaves began appearing after each write surge above in #2. These hit max_connections on all slaves simultaneously, and apaches went critical. Note that there were no slow queries involved; just an order of magnitude more connections and queries than normal.

5. Significant numbers of wikiadmin connections sat in SELECT MASTER_POS_WAIT due to #3, which reduced the available connections for #4.

6. We killed masses of wikiadmin and wikiuser sleeping connections to make way for new ones.

7. We stopped jobrunners.

8. Things recovered.

Observations and questions:

1. Batching the LinksUpdate DELETE and UPDATE queries would help with replag.

2. The storm of wikiuser traffic after the jobs was due to cache invalidation and presumably a lot of duplicated effort? Could that be mitigated in another layer above the DBs?

3. Can we throttle jobrunners more, or make them smarter in these situations?