ESA FAQ: What is Resource Conservation mode on the ESA?

Introduction

This document describes what Resource Conservation mode on the Cisco Email Security Appliance (ESA) is.

What is Resource Conservation mode on the ESA?

Resource Conservation mode is a self-protective mode that the ESA  enters when it is short on internal system resources. For example, when RAM utilization goes above 75% of capacity, AsyncOS will enter into Resource Conservation mode.  A shortage of any of several resources, including queue space, can cause the system to enter into Resource Conservation mode.

Fundamentally, Resource Conservation mode is entered when the rate of messages coming into an ESA is higher than the rate of messages leaving the appliance. Although differentials in these rates are common, if the incoming rate exceeds the delivery rate for a long period of time or by a huge differential, the appliance does not have the resources to store additional messages.

The goal behind Resource Conservation mode is to slow down traffic coming into the ESA so that the delivery rate can exceed the acceptance rate and drain messages out of the system.

Resource Conservation mode is not an on/off switch. When AsyncOS enters Resource Conservation mode, it gradually slows down the rate at which it will accept messages to try and balance the incoming and outgoing message rates. In extreme cases where resources are totally exhausted, AsyncOS will refuse to accept new messages.

When AsyncOS enters Resource Conservation mode, it sends an alert message. If you have configured alerts using alertconfig, you will receive a copy of this alert.

Note: The alert does not go through the normal AsyncOS mail system, and thus can be delivered even when other message delivery is halted. When AsyncOS leaves Resource Conservation mode, you will receive another alert.

You can see Resource Conservation mode in both the GUI and in the CLI. 

GUI

From the ESA GUI, navigate to Monitor > System Status in order to view the Resource Conservation mode status.

CLI

Enter the status detail command into the ESA CLI in order to verify whether an appliance has entered Resource Conservation mode. 

The Resource Conservation value shown in both the GUI and CLI is a value between 0 and 60, or the number 999. Numbers from 0 to 60 represent the degree to which the system is decreasing its acceptance of messages in order to prevent the rapid depletion of critical system resources. Higher numbers represent a higher degree of decreased acceptance. Zero represents no decrease in acceptance, a system operating at its full acceptance rate. If the Resource Conservation value shows 999, the system will accept no injected messages.