Mastering Ansible

Chapter 1
Task performance
Doing the math from the above description, that's at least three SSH connections
per task, per host. In a small fleet with a small number of tasks, this may not be a
concern; however, as the task set grows and the fleet size grows, the time required to
create and tear down SSH connections increases. Thankfully, there are a couple ways
to mitigate this.
The first is an SSH feature, ControlPersist, which provides a mechanism to create
persistent sockets when first connecting to a remote host that can be reused in
subsequent connections to bypass some of the handshaking required when creating
a connection. This can drastically reduce the amount of time Ansible spends on
opening new connections. Ansible automatically utilizes this feature if the host
platform where Ansible is run from supports it. To check whether your platform
supports this feature, check the SSH main page for ControlPersist.
The second performance enhancement that can be utilized is an Ansible feature
called pipelining. Pipelining is available to SSH-based connection methods and is
configured in the Ansible configuration file within the ssh_connection section:
[ssh_connection]
pipelining=true
This setting changes how modules are transported. Instead of opening an SSH
connection to create a directory, another to write out the composed module, and a
third to execute and clean up, Ansible will instead open an SSH connection and start
the Python interpreter on the remote host. Then, over that live connection, Ansible
will pipe in the composed module code for execution. This reduces the connections
from three to one, which can really add up. By default, pipelining is disabled.
Utilizing the combination of these two performance tweaks can keep your playbooks
nice and fast even as you scale your fleet. However, keep in mind that Ansible will
only address as many hosts at once as the number of forks Ansible is configured
to run. Forks are the number of processes Ansible will split off as a worker to
communicate with remote hosts. The default is five forks, which will address up to
five hosts at once. Raise this number to address more hosts as your fleet grows by
adjusting the forks= parameter in an Ansible configuration file, or by using the –
forks (-f) argument with ansible or ansible-playbook.
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