As you may have heard, on 30 June 2015, the last minute of the day 23:59 UTC will last 61 seconds instead of 60 seconds. The reason for this leap second is to sync time with the rotation of our Earth. The previous leap second was added in 2012, websites like LinkedIn, Mozilla and Reddit went down due to this leap second.
Because a progressive number of computer systems rely on time synchronization, this means extra work for System Administrators (that will take much longer than 1 second). On the other hand there is also a lot of discussion; are leap seconds really useful in a world that relies on computer systems?
That brought me to the question; “Are VMware products affected and what can we do to prevent misery?”
At this moment VMware’s download page shows a warning and a reference to KB “Support for Leap Seconds in VMware Products (2115818)”
In essence, only products configured as NTP (Network Time Protocol) clients are potentially affected. Although ESX and ESXi utilize a NTP client, fortunately these products are not affected; this was also evidenced during previous occurrences in 2008 and 2012. See KB “VMware ESXi and Leap Seconds (2121190)” for more information.
KB “VMware products unaffected by the Leap Second change (2121624)”, presents an overview of products not affected by the leap second, but it is advised to read through KB “Support for Leap Seconds in VMware Products (2115818)” previously mentioned, for a complete overview. Some older product versions are affected. E.g. the latest versions of the vCenter Server Appliance are fine; however versions 5.0, 5.0 U1b, 5.1.0a and 5.1.0b are based on Linux kernels which need additional measures to prevent damage.
A measure often mentioned is to enable Slew mode for NTP. This is described in KB “Enabling Slew Mode for NTP (2121016)“. Slew mode should be enabled 24 hours prior to the leap second and continued for 48 hours afterwards (adjust difference with your local time zone!).
What is this slew mode? By putting the ntpd daemon in Slew mode, time adjustments will be made in very small steps instead of a sudden change. For more information read the official description of the ntpd daemon and this discussion.
Also keep in mind that guest operating systems can also be affected by the leap second issue. So do some research for the OS.
If you wonder how your vCenter Server on Windows handles leap seconds? You may find some info in this comprehensive KB 909614 (How the Windows Time service treats a leap second) from Microsoft.
As always, I thank you for reading and wishing you good preparations for the next leap.