Expert Oracle Exadata - Parent Directory

CHAPTER 12 MONITORING EXADATA PERFORMANCENow let’s look at some of the individual metrics we have put onto the page, as shown in Figure12-15.Figure 12-15. Various celldisk average metrics charts displayed in Grid ControlIn the Avg Celldisk Small Read Latency (msec) chart here, we do see an issue. An hour before 12PM, theaverage small block I/O (under 256kB) latency has jumped up, probably because more large I/Ooperations were done in the cell. We can conclude this from the other two metrics in the top row; theAvg Cellidisk Read Requests has significantly dropped, while the Avg Celldisk Reads (MB) has remainedthe same, or actually even increased slightly. So there are more large I/Os being done, perhaps due tomore queries using a Smart Scan. The small I/O latency has suffered because of this. Your OLTPdatabase users running on the same cluster may notice that.This is a place where you should find out what causes all this extra I/O. We will look at it later in thischapter. If you want to avoid situations where large Smart Scans impact small (single-block) IOs, youshould consider enabling the I/O Resource Manager (IORM), which allows you to set the IORM goal tooptimize random I/O latency as opposed to full scanning throughput. Additionally, you could make surethat your hot segments accessed with single-block reads are cached in flash cache. See Chapter 7 formore about IORM and resource management.Figure 12-16 shows an example of the total cell disk I/O throughput chart. Note that the metric usedstarts with “Total,” indicating that the throughput MB values are summed over all twelve disks in the cell(as opposed to the Average metrics, which show averages across the disks). This is a very importantpoint. If you are summing together average metrics like in the Avg Celldisk Reads section (top rightcorner) of Figure 12-15, then you will still end up with a sum of the average cell disk read rates, not thetotal aggregate throughput of all disks in the cell. The cell disks’ average metrics are good for chartingthings like the min, max, and cell-level average I/O latencies or disk utilization figures, but not valid formonitoring things like I/O throughput of all cells.402