“Although we use MySQL’s transactional InnoDB tables, they can still sometimes be left in an unrecoverable state,” WikiPedia staff added. “Attempting to bring up the master database and one of the slaves immediately after the downtime showed corruption in parts of the database.”
Last month the LiveJournal weblog service suffered an extended outage in similar circumstances, with a data center power triggering a lengthy recovery of large volumes of data in MySQL databases.
For both LiveJournal and Wikipedia, the recovery process coincided with posts on Slashdot, which have been known to overwhelm sites with traffic. Not to fear – the Wikipedia entry on the Slashdot effect was online and available last night, even as other parts of Wikipedia remained offline.