IL-0320 Postmark · 2011-10-20 (published)database
Postmark's primary MySQL disk filled but MySQL kept running, so MySQL-MMM never failed over and sending was delayed
Postmark runs MySQL in a high-availability setup with MySQL-MMM, which usually detects a failing server and fails over automatically. At about 8:15am EDT the disk of the primary MySQL server filled much faster than normal and ran out of space, and email sending, the API and the web interface were delayed. MySQL did not stop; it only had trouble writing some data to disk, so MySQL-MMM saw no reason to fail over. The team first tried to free up logs and space, and after about 30 minutes, having wanted to confirm that replication was healthy first, failed over manually to the second server. Service returned to normal and the team recovered the failed jobs. No email was lost, because when the database is unavailable each Postmark server queues emails locally and sends them after recovery.
- Severity
- –
- Scope
- Email sending, the API and the web interface were delayed; no emails were lost
- Users affected
- –
- Region
- –
- Time to detect
- –
- Time to resolve
- –
Symptoms
At about 8:15am EDT Postmark's primary MySQL server ran out of disk space, and email sending, the API and the web interface were delayed. Disk space alerts fired, but the space had grown much faster than usual and MySQL was already struggling when the alerts were noticed. MySQL kept running while it failed to write some data to disk, so MySQL-MMM, which normally detects problems and fails over to a healthy server, never triggered a failover; the team spent about 30 minutes trying to free logs and disk space before failing over by hand.
Root cause
Root cause Capacity exhaustion
- Trigger
- –
Detection
Per-server monitoring raised disk space alerts, but the disk filled so quickly that MySQL was already struggling by the time the alerts were noticed.
Remediation
- Tried to free up logs and disk space on the primary server
- Failed over manually to the second MySQL server
- Recovered the jobs that had failed
- Sent the emails that each server had queued locally while the database was unavailable
Action items
- Upgrade the MySQL cluster with a focus on availability and performance
- Make the alerts more sensitive to catch problems sooner
Lessons
Looking back, Postmark said it should have failed over to the second server right away instead of first making sure replication was healthy.
Sources
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