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Fundamentals of I/O benchmarking

Measure for Measure

Article from ADMIN 32/2016

By Thorsten Stärk , By Benjamin Schweizer

Admins often want to know how to measure the performance of a specific solution. Care is needed, however: Where there are benchmarks, there are pitfalls.

Administrators wanting to examine a specific storage solution will have many questions. To begin: What is storage performance? Most admins will think of several I/O key performance indicators (KPIs), for which the focus is on one or the other. However, these metrics describe different things: Sometimes they relate to the filesystem, sometimes to raw storage, sometimes to read performance, and sometimes to write performance. Sometimes a cache is involved and sometimes not. Moreover, the various indicators are measured by different tools.

Once you have battled through to this point and clarified what you are measuring and with which tool, the next questions are just around the corner: Which component is the bottleneck that is impairing the performance of the system? What storage performance does your application actually need? In this article, we will help you answer all these questions.

Fundamentals

An I/O request passes through several layers (Figure 1) in the operating system. These layers each build on one another. For example, the application and filesystem layer (with technologies such as LVM, DRBD, mdadm, multipathing, devmapper, etc.) is based on the block virtualization layer. Closer to the hardware, you will find the block layer, the SCSI layers, and finally the devices themselves (RAID controllers, HBAs, etc.).