Expert Oracle Exadata - Parent Directory

CHAPTER 11 UNDERSTANDING EXADATA PERFORMANCE METRICS Kevin Says: “Regarding passthrough functionality, the authors are correct in asserting that passthrough isgenerally a sign of problems. However, late releases of Exadata Storage Server software have a feature that triesto mitigate the imbalances between processor utilization in the storage cells and that of database hosts understorage-intensive processing. The definition of “a problem” is subjective. In situations where Smart Scan offloadprocessing is saturating the storage processors—thus creating a bottleneck for data flow—query completion timeis severely impacted. That is a problem. Solutions can, sometimes, be problems as well.In these more recent releases of cellsrv there is a hint about database grid processor utilization in the metadataof each Smart Scan I/O request passed over iDB to storage. If storage is significantly busier than the hosts (CPUwise),cellsrv will send data to the foreground processes after no, or very little, offload processing—dependingon whether there is HCC involved or not. The term passthrough refers to those times when Smart Scan chooses toskip offload processing. As of the publication of this book, the only reason Smart Scan chooses to skip offloadprocessing is for reasons of processor saturation. There may be other triggers in the future. The infrastructure is inplace.The authors (and I) try to impart the concepts about when a Smart Scan will occur and when a Smart Scan willneed to revert to block mode processing. Here, on the other hand, we have a case where Smart Scan simplychooses to cease offload processing for a significant percentage of the data being read from disk. This middleground, however, is not a fall-back to block mode processing, because the buffering is still PGA (direct pathreads). The term Oracle has chosen for this middle-ground sums it up the best: passthrough. To that end, wecould say there is now another type of scan: Partially Smart Scan.So, how much Smart Scan value remains when in passthrough mode? Well, it is still a Smart Scan, so storageindexes can still be of benefit. However, filtration will not occur in storage. Projection, on the other hand, will occurin storage only if the data being scanned is EHCC. You see, projecting EHCC tuples is very inexpensive and, as weknow, reducing payload is the critical service Smart Scan aims to deliver. So if payload reduction can occur, that isa good thing.So what does all this mean? Well, idle processors are a very bad thing with DW/BI workloads, so this new behaviorcould be a good thing. The only aspect of the feature that makes me wary is that factors external to the query canchange the runtime dynamic. Consider, for instance, a query with a good execution plan that takes 10 seconds tocomplete when processing without other concurrent queries. Suppose further that this query burdens the storageprocessors to some 75 percent. When executed concurrently with another query that utilizes the remaining 25percent storage processor bandwidth, Smart Scan will stop offload processing for some amount of the data readfrom disk—for both of these queries. This is a case of hysteresis.366