Expert Oracle Exadata - Parent Directory

CHAPTER 10 EXADATA WAIT EVENTSYou’ll notice that the direct path read event (which cell smart table scan replaces) providesadditional information including the file number, the offset into the file (first dba) and the number ofcontiguous blocks read (block cnt). On the other hand, with the direct path read event, there is noindication how the read requests are routed to the individual cells.■ Note: The Cell hash number reported with many of the wait events can be found in the V$CELL view. This viewhas only two columns, CELL_PATH and CELL_HASHVAL. The CELL_PATH column actually contains the IP address ofthe storage cell.cell smart index scanTime is clocked to the cell smart index scan event when fast full index scans are performed that areOffloaded. This event is analogous to cell smart table scan, except that the object being scanned is anindex. It replaces the direct path read event and returns data directly to the PGA of the requestingprocess as opposed to the buffer cache.Event MeaningThis event does not show up very often on the systems we have observed, probably for several reasons:• Exadata is quite good at doing full table scans and so the tendency is to eliminate alot of indexes when moving to the platform.• Direct path reads are not done as often on index scans as they are on table scans.One of the important changes to Oracle 11.2 is the aggressiveness with which itdoes direct path reads on serial table scans. This enhancement was probablypushed forward specifically in order to allow Exadata to do more Smart Full TableScans, but regardless of that, without this feature, only parallel table scans wouldbe able to take advantage of Smart Scans. The same enhancement applies to indexfast full scans. That is, they can also be done via serial direct path reads. However,the algorithm controlling when they happen appears to be less likely to use thistechnique with indexes (probably because the indexes are usually much smallerthan tables).• In addition, only fast full scans of indexes are eligible for Smart Scans (range scansand full scans are not eligible).326