Smart Flash Cache and Smart Flash Logging - Oracle Exadata Recipes: A Problem-Solution Approach - page 527

This output shows us the following information:

• For our index (

DATA_OBJECT_ID=3080102 ), 3 flash cache misses were registered on cell
cm01cel02 and a total of 98,304 bytes of data was written to our flash storage.
For the table ( •

DATA_OBJECT_ID=3010199 ), 1 flash cache miss was observed on cm01cel02 for a
total of 32,768 bytes.

• The storage cells recorded zero flash cache hits; this was expected because, at the time the

query was executed, flash cache had not been populated.

• The total number of flash cache misses equaled 4: three for the index and 1 for the table. The

count matches our total expected physical I/O count based on the execution plan listed above
and the height of our index (I/Os required = DBA_INDEXES.BLEVEL + index leaf node blocks +
table blocks = 2 + 1 + 1 = 4):

SQL> select index_name,blevel
2 from dba_indexes
3 where index_name='RCV_TRANSACTIONS_U1';
INDEX_NAME BLEVEL
------------------------------ ----------
RCV_TRANSACTIONS_U1 2

The main thing to understand from this test is that uncached physical reads, if Exadata deems suitable, will
populate data in flash cache.
What happens if we execute the same query again after flushing our database buffer cache? Following are the
system statistics and flashcachecontent statistics on the next execution:

Statistic Value
---------------------------------------- ----------------
cell flash cache read hits 6
consistent gets 7
db block gets 0
physical read IO requests 6
physical read total bytes 49152
physical reads 6
session logical reads 7
[oracle@cm01dbm01 ]$ dcli -g ~/cell_group cellcli -e list flashcachecontent attributes
dbUniqueName,hitCount,missCount,cachedSize,objectNumber where objectNumber=3080102
cm01cel02: VISX 3 3 98304 3080102

[oracle@cm01dbm01 ]$ dcli -g ~/cell_group cellcli -e list flashcachecontent attributes
dbUniqueName,hitCount,missCount,cachedSize,objectNumber where objectNumber=3010199
cm01cel02: VISX 1 1 32768 3010199

[oracle@cm01dbm01 ]$

We can see that each of our physical read I/O requests was satisfied from flash cache; both the cell flash cache
read hits and physical read IO requests statistics showed a value of 6. Additionally, our flashcachecontent
statistics shows three flash cache hits for our index and one for our table, and zero flash cache misses as compared to
the initial execution.