In this Document

Applies to:

Purpose

Hangcheck_timer module is required to run a supported configuration in Oracle Real Application Clusters environments on Linux, with Oracle releases 9i, 10g, or 11gR1 RAC.  This note identifies and outlines the requirements needed to configure hangcheck-timer in an Oracle Enterprise Linux, Red Hat Linux, or SUSE Linux environment.

Scope

This article is provided for product management, system architects, and system administrators involved in deploying and configuring Oracle RAC 9i, 10g, or 11gR1 in a Linux environment. This document will also be useful to field engineers and consulting organizations to facilitate installations and configuration requirements of Oracle in a Linux RAC environment.

Details

Starting in release 9.2.0.2 and later, Oracle RAC environments required using a new I/O fencing model, named the hangcheck-timer module. This module was implemented to replace the Watchdog module, which provided similar fencing functionality. Hangcheck-timer was subsequently delivered as part of the standard kernel distribution for Linux kernel releases 2.4 and above.

Hangcheck-timer should be loaded at boot time, and monitors the Linux kernel for long operating system hangs that could affect the reliability of a RAC node.  It runs in kernel mode and uses the Time Stamp Counter (TSC) to catch scheduling delays or node hangs.  This is done by setting a timer, then checking when the timer fires as to whether it was delayed by more than the allowed margin of error.  If the duration exceeds the allowed time of (hangcheck_tick + hangcheck_margin seconds), the machine is restarted.  Hangcheck-timer will not cause reboots to occur due to CPU starvation.

 Hangcheck-timer requires three configuration parameters:

• hangcheck_tick - defines how often, in seconds, the hangcheck-timer checks the node for hangs. The default value is 60 seconds.
• hangcheck_margin - defines how much margin is allowed, in seconds, between expected scheduling and real scheduling time. The default value is 180 seconds.
• hangcheck_reboot - determines if the hangcheck-timer restarts the node if the kernel fails to respond within the sum of the hangcheck_tick and hangcheck_margin parameter values. If the value of hangcheck_reboot is equal to or greater than 1, then the hangcheck-timer module restarts the system. If the hangcheck_reboot parameter is set to zero, then the hangcheck-timer module will not reboot the node, even if a hang is detected.   The default value varies by kernel version.  In the 2.4 kernel, the default is 1.  In 2.6 kernels, the default is 0.

• 9i: Assuming the default setting of "oracm misscount" is set to 220 seconds:
  hangcheck_tick=30 hangcheck_margin=180 hangcheck_reboot=1
• 10g/11gR1: Assuming the default setting of "CSS misscount" is set to either 30 or 60 seconds:
  hangcheck_tick=1 hangcheck_margin=10 hangcheck_reboot=1

You must always ensure that the Cluster misscount setting is greater than the sum of the setting for hangcheck_tick + hangcheck_margin.

@  Unpublished information for Oracle Support Internal Use:

When running Oracle Clusterware on Linux, hangcheck-timer should always be configured on each RAC cluster node, as the functionality of this module is required to provide I/O Fencing to ensure no stray writes will occur from an evicted node in a RAC cluster.  To verify if the hangcheck-timer module is running on a node execute as the root or oracle user: