Configuring RAID for Optimal Performance

Configuring RAID for Optimal Performance Performance Measurement Tools

2.3 IOmeter*

IOmeter* (www.iometer.org) is a sophisticated tool that can measure RAID performance,

including sequential, random and mixed workloads, adjustable block sizes, and queuing. This

tool requires certain level of proficiency to use it.

Unfortunately, queuing does not work under Linux with the current IOmeter version 2006.07.27.

One of the common mistakes made with IOmeter is setting the access type to mixed (random +

sequential). This selection will have big impact on performance measured in MB/s.

For achieving maximum MB/s numbers, 100% sequential (0% random) access pattern must be

used with a large enough block size (between 64KB and1MB). Also the number of Outstanding

I/Os (queue depth) typically should be in the 8-128 range.

The following formula provides guidance for selecting optimal outstanding I/Os and block size

values:

Outstanding_IOs x Block_Size = 2 x Strip_Size x Number_of_HDDs_in_Stripe

The number of HDDs in stripe does not include parity HDDs. For example, RAID5 array

with 9 HDDs will have 8 HDDs in the stripe.

2.4 Measuring Performance under Linux

The Linux* OS has a substantially different I/O queuing model (asynchronous I/O) than the

Microsoft Windows* OS. We are not aware of any performance measurement tools that support

I/O queuing for read operations under Linux. When measuring sequential read throughput under

Linux, it is important to enable Read Ahead mode (either Adaptive or Always).