LBaaS back-end pool connection limit is 10% of listener connection limit for reference and namespace drivers

[Stephen Balukoff]

Both the reference Octavia driver and the namespace driver use haproxy to deliver load balancing services with LBaaSv2. When closely looking at the operation of the haproxy daemons with a utility like hatop ( https://github.com/feurix/hatop ), one can see that the connection limit for back-ends is exactly 10% of whatever the connection limit is set for the pool's listener front-ends. This behavior could cause an unexpectedly low effective connection limit if the user has a small number of back-end servers in the pool.

From the haproxy documentation, this is because the default value of a backend's "fullconn" parameter is set to 10% of the sum of all front-ends referencing it. Specifically:

"Since it's hard to get this value right, haproxy automatically sets it to
10% of the sum of the maxconns of all frontends that may branch to this
backend (based on "use_backend" and "default_backend" rules). That way it's
safe to leave it unset."

(Source: https://cbonte.github.io/haproxy-dconv/configuration-1.6.html#fullconn )

The point of this calculation (according to the haproxy documentation) is to protect fragile back-end servers from spikes in load that might reach the front-ends' connection limits. However, for long-lasting but low-load connections to a small number of back-end servers through the load balancer, this means that the haproxy-based back-ends have an effective connection limit that is much smaller than what the user expects it to be.

[Stephen Balukoff (sbalukoff)]

The Octavia (reference) portion is being worked on here: https://review.openstack.org/#/c/369102/

[Gregory Thiemonge (gthiemonge)]

Abandoned after re-enabling the Octavia launchpad.