AIX使用异步I/O(aio)提高系统性能

qianwei01

如果是同步I/O,当一个I/O操作执行时,应用程序必须等待,直到此I/O执行完.  \r\n     相反,异步I/O操作在后台运行,I/O操作和应用程序可以同时运行,提高了系统性能.  \r\n     使用异步I/O会提高I/O流量,如果应用是对裸设备进行操作,这种优势更加明显.  \r\n     因此象数据库,文件服务器等应用往往会利用异步I/O,使得多个I/O操作同时执行.  \r\n      \r\n     1. 如何知道是否需要异步I/O?  \r\n     * 执行命令:\"vmstat # #\" 如果\"wa\"值超过25%.  \r\n     * 执行命令:\"iostat # #\" 如果\"%tm_act\"值超过35%.  \r\n     2. 查询有几个AIO服务器: pstat -a | grep aios | wc -l  \r\n     3. 应该设几个AIO服务器?  \r\n     一般为硬盘数的10倍,但不要超过80.  \r\n     4. AIO驱动程序: bos.rte.aio  \r\n     5. 配置AIO为可用: # smit chgaio ->  \r\n     STATE to be configured at system restart [available]  \r\n

ArthurXu

好像aio只有对文件系统才有效，裸设备是自动异步的。\r\n  \r\n    \r\nBookmark Fixed font  Go to End \r\n\r\nDoc ID:  Note:70161.1 \r\nSubject:  WHEN TO USE AIO MIN AND MAX SERVERS ON AIX \r\nType:  BULLETIN \r\nStatus:  PUBLISHED \r\n Content Type:  TEXT/PLAIN \r\nCreation Date:  24-MAY-1999 \r\nLast Revision Date:  05-JAN-2001 \r\n \r\n\r\nWhen to use Asynchronous IO (\'AIO\') min and max servers on AIX  \r\n===============================================================\r\n\r\nWhen should you set Asynchronous io (\'AIO\') min and max servers?\r\nThe answer is different depending on whether you use a file system,\r\nraw devices alone, or raw devices with VSDs or HSDs.  The answer \r\nalso changes depending on what version of AIX you are running and\r\nwhether or not 4.3 APAR IX79690 is applied.\r\n\r\n\r\nAsynchronous IO (\'AIO\') on AIX can be handled two different ways:\r\n\r\n1. by aio servers, or\r\n2. directly by the kernel (aka fastpath aio)\r\n\r\nIf AIO is handled by the kernel, you do not need to worry about \r\ntuning AIX minservers and maxservers.\r\n\r\nAIX Version     File type       AIO method\r\n-----------     -----------     -------------\r\n4.2.1           filesystem      aio server\r\n4.2.1           raw device      kernel\r\n4.2.1           VSD/HSD         aio server\r\n4.3             filesystem      aio server\r\n4.3             raw device      kernel\r\n4.3             VSD/HSD         kernel  (with APAR IX79690)\r\n4.3.2           filesystem      aio server\r\n4.3.2           raw device      kernel\r\n4.3.2           VSD/HSD         kernel  (APAR IX79690 is in 4.3.2)\r\n4.3.3           filesystem      aio server\r\n4.3.3           raw device      kernel\r\n4.3.3           VSD/HSD         kernel\r\n\r\nExplanation: \r\n============ \r\n\r\nWhen you use a file system on AIX, kernel processes known as \r\nkprocs are used.  The kprocs require aio servers.  When you use\r\nraw devices kprocs are not used because AIO is handled directly \r\nby the kernel.  This is known as Fastpath AIO.  VSDs and HSDs \r\nare built on raw devices but they could not make use of Fastpath \r\nAIO prior to AIX 4.3 with APAR IX79690.\r\n\r\nWith APAR IX79690 Fastpath is on by default for raw devices with \r\nor without VSD/HSD.  The APAR will allow you to disable Fastpath.  \r\nHowever, there is no reason to do this unless you are on an old \r\nmodel machine which cannot handle Fastpath.  Prior to the APAR \r\nyou could not disable Fastpath for raw devices without VSD/HSD \r\nand you could not enable Fastpath for raw devices with VSD/HSD.\r\nThe APAR is included in 4.3.2.\r\n\r\nIf any request tried on Fastpath is not successful then the kproc \r\npath is tried (using AIO servers).  So if you adjust the aio servers \r\nand see a difference when you think you should be using Fastpath \r\nthen this indicates some of your fastpath requests are failing and \r\nare tried with the kproc path.\r\n.

larryh

如此说来，应该是：\r\n在使用文件系统的情况下，AIO server的最大和最小数量对性能有影响，必要时需要调整。\r\n在使用裸设备的情况下，不使用AIO server，于是其数量与裸设备IO性能无关。\r\n\r\n但是，AIO不打开（AIX默认不打开），裸设备一样用不成AIO，而不是自动就有AIO的。