Expert Oracle Exadata - Parent Directory

CHAPTER 11 UNDERSTANDING EXADATA PERFORMANCE METRICS--------------------------------------------------------------------------------------SID, USER , TYPE, STATISTIC, HDELTA--------------------------------------------------------------------------------------400, TANEL , STAT, physical read total bytes , 133.8G400, TANEL , STAT, cell physical IO interconnect bytes , 4.54G400, TANEL , STAT, cell smart IO session cache lookups , 130400, TANEL , STAT, cell smart IO session cache hits , 130400, TANEL , STAT, cell physical IO interconnect bytes returned by smart scan, 4.54G400, TANEL , STAT, table scans (long tables) , 132400, TANEL , STAT, table scans (rowid ranges) , 132400, TANEL , STAT, table scans (direct read) , 132400, TANEL , STAT, cell scans , 132cell smart IO session cache hitsThis statistic shows how many times a database session managed to reuse a previously initialized SmartScan session in the cell. This statistic shows up when a single execution plan scans through multiplesegments (like with partitioned tables) or revisits the same segment during a single execution.cell smart IO session cache lookupsThis statistic is incremented every time a database session tried to reuse a previously initialized SmartScan session in the cell. If the cell smart IO session cache hits statistic increments too, then the lookupwas successful and a previous session can be reused. The smart I/O session caching works only withinan execution (and subsequent fetches) of an open cursor. Once the execution finishes, the nextexecutions (even of the same cursor) would have to set up new Smart Scan sessions (and communicatethe new consistent read snapshot SCN to the cells too).cell transactions found in commit cacheThis statistic is related to the consistent read mechanism Oracle has to guarantee, even on Exadata. Itshows how many times the Smart Scan sessions checked the cell commit cache to decide whether a CRrollback is needed or not; and it finds the transaction status information in the cell commit cache. Thisavoids a round trip to the database layer to check that transaction’s status using undo data availablethere. There’s more information about how the consistent reads work with Exadata Smart Scans in thecell blocks helped by commit cache statistic section.chained rows processed by cellBefore explaining what this specific statistic means, let’s look at how Smart Scans deal with chainedrows. Chained rows pose a problem for Smart Scans. The chained row’s “next” row pieces may beanywhere in the segment; and thanks to the ASM striping, it’s not guaranteed that the next row pieces ofa chained row are in the same cell where the row head piece is. So a chained row may be physically splitacross multiple different cells. Given that cells never speak to each other (for scalability and simplicity),how would you be able to construct the full row when needed? The way cellsrv currently solves thisproblem is that whenever the Smart Scan hits a chained row (and realizes it has to fetch its next row369