As the services will be failed over to the other SP, there will be service disrruption for about 1~2 minutes. Also both the SPs will be rebooted one by one. During this stage, it is only recommended to keep I/O to array to minimum and ensure your hosts are configured properly to tolerate this 1~2 minute of storage unavailbility.  For example if someone is copying hundreds of gigs of files to a CIFS share and you reboot the SP, that copy operation will fail.  I would like to also add that, from VNXe OE version 2.3.1 onwards it will not reboot the SPs when you change the NTP settings. So, I recommend you to upgrade to recent code release MR4 - 2.4.0.
I'm setting the time on my VNXe 3100, and it ask me "Are you ready to proceed with the system reboot?"  Are there any steps I need to perform before I select "yes"?.  Anything that needs to be brought down first?
How to reboot the VNXe 3100?