如何启用 DIRECT_IO ？

什么是DIRECT_IO？

This is an onconfig parameter that allows Informix to override the cache layer of file systems,

so IDS writes and reads using the Kernel Asynchronous IO bypassing the filesystem's caching mechanism.

This offers virtually the same benefits as if one is using raw devices,

namely safely writing directly to the disks, and a performance gain from using Kernel IO. 

如下四个测试场景：

1. Creation of a dbspace on a linux ext4 file system (journaled), with the onconfig parameter DIRECT_IO set to 0, then dbimport the data into a non-Logged database.
2. Creation of the dbspace on a linux ext4 file system (journaled), with the onconfig parameter DIRECT_IO set to 1, then dbimport the data into a non-Logged database.
3. Creation of the dbspace on a Linux ext2 file system (not journaled), with the onconfig parameter DIRECT_IO set to 0, then dbimport the data into a non-Logged database.
4. Creation of dbspace on a Linux ext2 file system (not journaled), with the onconfig parameter DIRECT_IO set to 1, then dbimport the data into a non-Logged database.

测试结果如下：

 To make this short: 
Test 1: execution time =   91m 41s 
Test 2: execution time = 195m 57s 

Test 3: execution time =   54m 49s 
Test 4: execution time =   49m 54s 

由此可见：

       DIRECT_IO并非是一个绝对指标，需要与操作系统的配置有很大关系。

I would include EXT3 with EXT4 as being slower than EXT2 unless you turn off the data journaling leaving only meta-data journaling active.  

In addition, note that EXT4 and EXT3 with journaling set to write-back mode, are unsafe 。