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If you have already upgraded to jessie and have systemd as init, you can switch back with these commands:

apt-get install -y sysvinit-core
reboot
apt-get purge -y systemd

Running with sysvinit works better on my servers, as daemons have a chance of doing their own graceful shutdown before the network is brought down.

Before upgrading to jessie, create /etc/apt/preferences.d/use-sysvinit:

Package: systemd-sysv
Pin: release o=Debian
Pin-Priority: -1

See also: How to stay with sysvinit in Debian Jessie by Petter Reinholdtsen