Linux 平台下Oracle 9i/10g/11gR1 IO-Fencing 的hangcheck-timer 模块说明，Hangcheck-timer 是Linux 提供的一个内核级的IO-Fenc
一.官网的说明
参考MOS：
9i, 10g, and11gR1 RAC [ID 726833.1]
Hangcheck_timermodule is required to run a supported configuration in Oracle Real ApplicationClusters environments on Linux, with Oracle releases 9i, 10g, or 11gR1RAC. This note identifies and outlines the requirements needed toconfigure hangcheck-timer in an Oracle Enterprise Linux, Red Hat Linux, or SUSELinux environment.
Note : Hangheck timer is notrequired starting with Oracle Clusterware 11gR2
Starting in release 9.2.0.2and later, Oracle RAC environments required using a new I/O fencing model,named the hangcheck-timer module. This module was implemented to replace theWatchdog module, which provided similar fencing functionality. Hangcheck-timerwas subsequently delivered as part of the standard kernel distribution forLinux kernel releases 2.4 and above.
Hangcheck-timer shouldbe loaded at boot time, and monitors the Linux kernel for long operatingsystem hangs that could affect the reliability of a RAC node. It runs inkernel mode and uses the Time Stamp Counter (TSC) to catch scheduling delays ornode hangs. This is done by setting a timer, then checking when the timerfires as to whether it was delayed by more than the allowed margin oferror. If the duration exceeds the allowed time of (hangcheck_tick +hangcheck_margin seconds), the machine is restarted. Hangcheck-timer willnot cause reboots to occur due to CPU starvation.
Hangcheck-timer requiresthree configuration parameters:
(1) hangcheck_tick - defines howoften, in seconds, the hangcheck-timer checks the node for hangs. The defaultvalue is 60 seconds.
(2) hangcheck_margin - defines howmuch margin is allowed, in seconds, between expected scheduling and realscheduling time. The default value is 180 seconds.
(3) hangcheck_reboot - determinesif the hangcheck-timer restarts the node if the kernel fails to respond withinthe sum of the hangcheck_tick and hangcheck_margin parameter values. If thevalue of hangcheck_reboot is equal to or greater than 1, then thehangcheck-timer module restarts the system. If the hangcheck_reboot parameteris set to zero, then the hangcheck-timer module will not reboot the node,even if a hang is detected. The default value varies by kernelversion. In the 2.4 kernel, the default is 1. In 2.6 kernels, thedefault is 0.
当hangcheck_reboot=1并且满足下面的公式时，hangcheck-timer将reboot系统: system hang time > (hangcheck_tick + hangcheck_margin)
All hangcheck-timer defaultvalues should be explicitly overridden when loading the kernel module, based onthe Oracle release as follows:
hangcheck_tick=30hangcheck_margin=180 hangcheck_reboot=1
--9i: 假如"oracle misscount"的缺省设置是220秒，则hangcheck_tick=30hangcheck_margin=180 hangcheck_reboot=1
hangcheck_tick=1hangcheck_margin=10 hangcheck_reboot=1
--10g/11gR1: 假如"CSS misscount"的设置是30或者60秒，则hangcheck_tick=1hangcheck_margin=10 hangcheck_reboot=1
You must always ensure thatthe Cluster misscount setting is greater than the sum of the setting forhangcheck_tick + hangcheck_margin.
When running OracleClusterware on Linux, hangcheck-timer should always be configured on each RACcluster node, as the functionality of this module is required to provide I/O Fencingto ensure no stray writes will occur from an evicted node in a RACcluster. To verify if the hangcheck-timer module is running on a nodeexecute as the root or oracle user:
# /sbin/lsmod | grep hangcheck
hangcheck-timer 2672 0
If the hangcheck-timer moduleis loaded (running) you will see output similar to above. When hangcheck-timeris not loaded no output is generated, and the command prompt is returned to theuser.
In an Oracle Enterprise Linux,Red Hat 4/5, or SUSE 9/10 environment the hangcheck-timer module is loadedusing the modprobe command:
# modprobe hangcheck-timer hangcheck_tick=1 hangcheck_margin=10hangcheck_reboot=1
In order to ensure the moduleis loaded at boot time, you should also place the same command in the appropriatelocal command execution directory (e.g. /etc/rc.d/rc.local, or/etc/init.d/boot.local). In earlier releases, hangcheck-timer was loadedusing insmod in place of modprobe. Consult your release specific documentationto determine which initialization method is required.
当检测到故障时，Hangcheck-timer 将向系统消息日志提供消息记录，并且模块启动阳极重启：
(1) 当 Hangcheck-timer 重新启动时，它可能会在 /var/log/messages 中留下“Hangcheck:hangcheck is restarting the machine”消息。
(2) 如果您在 /var/log/messages 中看到以下消息：“Hangcheck:hangcheck value Pastmargin!”这意味着需要重新启动，但未执行，因为hangcheck_reboot 未设置为 1。如果看到此消息，您必须按照本说明前面所述重新加载hangcheck 模块，并将hangcheck_reboot 值设置为1。
注：
Bug:6125546，它可以阻止hangcheck-timer在RHEL4中重新启动（已在2.6.9.56或RHEL4.6中修复）
