Just under a year ago, on June 14, 2013, I reported MySQL Bug 69477 (and MariaDB Bug MDEV-4662) titled “InnoDB: Use of large externally-stored fields makes crash recovery lose data”. The gist of this bug is that if you use large BLOB fields (> ~10% of your InnoDB redo logs size), you can compromise crash recovery and suffer data loss after a crash.

The MySQL bug was marked as “Verified” in less than an hour (wow!), but since then there was not a single word about it. Suddenly a couple of days ago, I got an email with “[Com,Ver->Csd]” in the subject. My immediate thought was “Damn it, closed?! How can they close it?! It’s a bug!” — however, upon opening the email I noticed it was not closed as “Won’t fix”, but rather, it was fixed!

Daniel Price at Oracle left the following comment on the bug:

Fixed as of 5.6.20, 5.7.5, and here’s the changelog entry:

Redo log writes for large, externally stored “BLOB” fields could overwrite the most recent checkpoint. The 5.6.20 patch limits the size of redo log “BLOB” writes to 10% of the redo log file size. The 5.7.5 patch addresses the bug without imposing a limitation. For MySQL 5.5, the bug remains a known limitation.

The limitation has also been documented in the reference manual. The changes should appear soon, with the next published documentation build.

http://dev.mysql.com/doc/refman/5.1/en/innodb-restrictions.html
http://dev.mysql.com/doc/refman/5.5/en/innodb-restrictions.html

Thank you for the bug report.

Thanks, Oracle, for fixing¹ this bug! I look forward to seeing the fix in both 5.6 and 5.7. I was just thinking the other day that I might have to fix it myself, because it seemed like nobody was working on it.

¹ Maybe next time leave a comment that you’re working on it? Pretty please? :)