2019.02.27

## 问题

Nov 5 13:05:41 RedHat5 kernel: INFO: task oracle:22439 blocked for more than 120 seconds.
Nov 5 13:05:41 RedHat5 kernel: “echo 0 > /proc/sys/kernel/hung_task_timeout_secs” disables this message.


## 解决方法

echo 0 > /proc/sys/kernel/hung_task_timeout_secs


## 原因

This is a know bug. By default Linux uses up to 40% of the available memory for file system caching. After this mark has been reached the file system flushes all outstanding data to disk causing all following IOs going synchronous. For flushing out this data to disk this there is a time limit of 120 seconds by default. In the case here the IO subsystem is not fast enough to flush the data withing 120 seconds. This especially happens on systems with a lof of memory. The problem is solved in later kernels and there is not “fix” from Oracle. I fixed this by lowering the mark for flushing the cache from 40% to 10% by setting “vm.dirty_ratio=10″ in /etc/sysctl.conf. This setting does not influence overall database performance since you hopefully use Direct IO and bypass the file system cache completely.