As we learned in my last blog, we should start our server performance tuning by looking for system-level bottlenecks. This involves understanding exactly how data flows into and out of your platform - and to do this, you need a block diagram. A block diagram shows the major components on the server's motherboard and the paths between them. From a good block diagram you can derive the maximum data transfer rate (aka bandwidth or throughput) achievable as data flows along those paths.
I usually look at my block diagram before beginning system tuning in order to identify potential bottlenecks. But some people use them in parallel: they measure the bandwidth of various parts of the system and then confirm what they see using the block diagram. You can determine if various parts of your system are heavily stressed, bottlenecked, or lightly utilized. In general you want to trace the path from where data enters your server (NIC, HBA, etc) up to the processor and back to memory or out of the server. The paths connecting one component to another are commonly known as buses. For each bus, multiply the speed by the width to determine the maximum potential bandwidth.
Let's use the block diagram for the Intel S5400SF server board as an example. It has 2 FSBs, each capable of 1333 or 1600 Mega-Transfers/second (MT/s). Each transfer on the FSB is 64 bits (8 bytes), so 8 bytes * 1,600,000,000 transfers gives a maximum theoretical bandwidth of 12.8GB/s per FSB segment. Keep in mind though that in reality a bus will not achieve its theoretical maximum bandwidth - depending on the type of bus it will probably realize 66-80% of the possible throughput.
So, where do you find these diagrams? If you are using an Intel server platform, the block diagrams can usually be found in the technical product specification for each board. If you purchase a platform from one of our OEM partners, ask your salesperson where to get it.
Look at the maximum bandwidth achievable on each link your data will travel over to gain a deeper understanding of how your workload will run on your platform.
Keep watching The Server Room for information on the other 7 habits in the coming weeks.