SSH pipelining is an Ansible feature to reduce the number of connections to a host.
Ansible will normally create a temporary directory under ~/.ansible (via ssh), then for each task, copy the module source to the directory (using sftp or scp) and execute the module (ssh again).
With pipelining enabled, Ansible will connect only once per task using ssh to execute python, and write the module source to its stdin. Even with persistent ssh connections enabled, it's a noticeable improvement to make only one ssh connection per task.
Unfortunately, pipelining is disabled by default because it is incompatible with sudo's requiretty setting (or su, which always requires a tty). This is because of a quirk of the Python interpreter, which enters interactive mode automatically when you pipe in data from a (pseudo) tty.
Update 2015-11-18: I've submitted a pull request to make pipelining work with requiretty. The rest of this post still remains true, but if the PR is merged, the underlying problem will just go away.
Pipelining can be enabled globally by setting “pipelining=True” in the ssh section of ansible.cfg, or setting “ANSIBLE_SSH_PIPELINING=1” in the environment.
With Ansible 2 (not yet released), you can also set ansible_ssh_pipelining in the inventory or in a playbook. You can leave it enabled in ansible.cfg, but turn it off for some hosts (where requiretty must remain enabled), or even write a play with pipelining disabled in order to remove requiretty from /etc/sudoers.
- lineinfile: dest: /etc/sudoers line: 'Defaults requiretty' state: absent sudo_user: root vars: ansible_ssh_pipelining: no
The above lineinfile recipe is simplistic, but it shows that it's now possible to disable requiretty, even if it's by replacing /etc/sudoers altogether.
Note the use of another Ansible 2 feature above: vars can also be set for individual tasks (and blocks), not only plays.