I use multiserver configuration 2xLdap, 2xStore, 2xMTA. All servers it's a xen paravirt machine.

my steps on each server:
  • root: /etc/init.d/zimbra stop
  • root: mv /etc/init.d/zimbra /root/
  • root: do-release-upgrade

  • remove old sar files - Y
  • restart services automatically -Y
  • replace /etc/securetty -Y
  • replace /etc/sysctl -Y
  • replace ssh config -Y
  • replace ntp config -Y
  • replace sudo -N
  • Grub - no checkbox, just press ok
  • reboot -N

Because Xen pygrub have some problem with work after upgrade "error boot loader didn't return any data" I made this:
  • aptitude -y purge grub2-common grub-pc grub-pc-bin
  • aptitude install grub
  • grub-mkdevicemap
  • grub-update
  • check the menu.lst file and reboot

After reboot
  • mv /root/zimbra /etc/init.d/
  • aptitude install libperl5.14
  • run install.sh

WARNING you can't run database check

Sequence of upgrade:
  • ldap1 (after upgrade /opt/bin/zmcertmgr deploycert self)
  • ldap2
  • mta1, mta2
  • stor1, stor2

after upgrade on ldap1 /opt/bin/zmcertmgr deploycert self --allserver and restart all zimbra's