queue_depth性能参数的设置

   在IBM的红皮书中对queue_depth参数的设置有公式可以依据，queue_depth参数合适与否对系统的性能起到重大的作用，尤其是在使用数据库应用的时候。以下是从IBM红皮书中摘到的部分内容。

The queue depth value cannot exceed the storage server maximum of 2048 for DS4000

storage servers running firmware 6.1x and later; and a maximum of 512 for firmware 5.3x and 5.4x. All logical drives on the storage server must share these queue limits. Some hosts define the queue depth only at the HBA level, while others may also define this limit at the storage device level, which ties to the logical drive. The following formulas can be used to determine a good starting point for your queue depth value on a per logical drive basis:

For firmware level 6.1x and higher: 2048 / (number-of-hosts * logical drives-per-host), and for firmware level 5.3 and 5.4: 512 / (number-of-hosts * logical drives-per-host) As an example: A storage server with 4 hosts with 12, 14, 16 and 32 logical drives attached respectively would be calculated as follows:

2048 / 4 * 32 (largest number of logical drives per host) = 16

512 / 4 * 32 (largest number of logical drives per host) = 4

If configuring only at the HBA level, you can use the formula: 2048 / ( total

number-of-HBA’s ), and 512 / ( total number-of-HBAs ) for the respective firmware

levels.

补充一下更改该参数的命令：

加入根据公式计算得到的值是4，可以通过下面的命令进行设置，在调整之前最好使用filemon工具收集统计信息，已备与调整后的比较。

#chdev -l hdiskX -a queue_depth=4 -P

尽管有公式可以遵循，假如“largest number of logical drives per host”发生了变化，是否还要重新进行调整，这样不仅管理上带来了麻烦，而且调整后的值对其他主机上的应用会产生什么样的影响，还得需要评定，所以对该公式的稳定性持有怀疑性。

在调整该参数时还要考虑，文件系统或者逻辑卷提供的主要服务是哪些，读写的侧重点。