Don’t Let a Leap Second Leap on Your Database!
Don’t Let a Leap Second Leap on Your Database!
In 2012, a leap second killed performance on plenty of popular sites. That's been largely remedied, but you can still double check to see if anything will break.
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This blog discusses how to prepare your database for the new leap second coming in the new year.
At the end of this year, on December 31, 2016, a new leap second gets added. Many of us remember the huge problems this caused back in 2012. Some of our customers asked how they should prepare for this year’s event to avoid any unexpected problems.
It’s a little late, but I thought discussing the issue might still be useful.
The first thing is to make sure your systems avoid the issue with abnormally high CPU usage. This was a problem in 2012 due to a Linux kernel bug. After the leap second was added, CPU utilization sky-rocketed on many systems, taking down many popular sites. This issue was addressed back in 2012, and similar global problems did not occur in 2015 thanks to those fixes. So it is important to make sure you have an up-to-date Linux kernel version.
It’s worth knowing that in the case of any unpredicted system misbehavior from the leap second problem, the quick remedy for the CPU overheating was restarting services or rebooting servers (in the worst case).
(Please do not reboot the server without being absolutely sure that your serious problems started exactly when the leap second was added.)
The following are examples of bug records:
- https://rhn.redhat.com/errata/RHBA-2012-0521.html
- https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1020285
The second thing is to add proper support for the upcoming event. Leap second additions are announced some time before they are implemented, as it isn’t known exactly when the next one will occur for sure.
Therefore, you should upgrade your OS tzdata package to prepare your system for the upcoming leap second. This document shows how to check if your OS is already “leap second aware”:
zdump -v right/America/Los_Angeles | grep Sat.Dec.31.*2016A non-updated system returns an empty output. On an updated OS, you should receive something like this:
right/America/Los_Angeles Sat Dec 31 23:59:60 2016 UTC = Sat Dec 31 15:59:60 2016 PST isdst=0 gmtoff=-28800
right/America/Los_Angeles Sun Jan 1 00:00:00 2017 UTC = Sat Dec 31 16:00:00 2016 PST isdst=0 gmtoff=-28800If your systems use the NTP service though, the above is not necessary (as stated in https://access.redhat.com/solutions/2441291). Still, you should make sure that the NTP services you use are also up-to-date.
With regards to leap second support in MySQL there is nothing to do, regardless of the version. MySQL doesn’t allow an extra second numeration within the 60 seconds part of timestamp datatype, so you should expect rows with 59 instead of 60 seconds when the additional second is added, as described here: https://dev.mysql.com/doc/refman/5.7/en/time-zone-leap-seconds.html
Similarly, MongoDB expects no serious problems either.
Let’s “Smear” the Second
Many big Internet properties, however, introduced a technique to adapt to the leap second change more gracefully and smoothly, called Leap Smear or Slew. Instead of introducing the additional leap second immediately, the clock slows down a bit, allowing it to gradually get in sync with the new time. This way there is no issue with extra abnormal second notation, etc.
This solution is used by Google, Amazon, Microsoft, and others. You can find a comprehensive document about Google’s use here: https://developers.google.com/time/smear
You can easily introduce this technique with the ntpd -x or Chronyd slew options, which are nicely explained in this document: https://developers.redhat.com/blog/2015/06/01/five-different-ways-handle-leap-seconds-ntp/.
Summary
Make sure you have your kernel up-to-date, NTP service properly configured and consider using the Slew/Smear technique to make the change easier. After the kernel patches in 2012, no major problems happened in 2015. We expect none this year either (especially if you take time to properly prepare).
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