Often, when invoking cellcli utility in exadata storage server,you see a ratio banner called cell efficiency ratio(CER) after copyright message.Have you ever think of what it is and why has it been displayed everytime?
I see this banner everytime and ignore it because the ratio value is always small.But suddenly , i saw a huge rise in the CER due to some reason.I know that before i see this rise ,i had offloaded lot of sql queries with smart scan,storage index,HCC,flash cache keep etc.. This is the main reason for the huge increase in CER
To define CER, it is the ratio between the data touched on the storage tier and data processed on the database tier
CER = Data touched on storage cell/Data processed by database tier
In above picture,CER is 489 which means most data is processed by database itself or to be descriptive,less data is offloaded for smart scan,storage index is not efficiently used and touched from storage layer
CellCLI> list cell attributes offloadEfficiency 489.3
Dont get frustated by seeing the low efficiency value but be happy that you dont have any negative values over there!Because negative value indicate that you dont use exadata upto the optimum level for what you pay
Though, CER is no longer increased in one day or night.The value gets better gradually on offloading data.
So always try to offload most of the oltp or batch workloads to the cell storage server and gradually increase CER for which you get blistering performance
The above output shows pretty impressive cell efficiency rate which shows that cell storage server is optimally used
CellCLI> list cell attributes offloadEfficiency 16,291.9
Lot of objects are kept in flashcache to increase CER.For example, look the following output
The more objects you keep in flash memory,more IO is reduced for database
CellCLI> LIST FLASHCACHECONTENT detail cachedKeepSize: 0 cachedSize: 65536 dbID: 3727006149 dbUniqueName: EXDBX hitCount: 2 missCount: 0 objectNumber: 87176 tableSpaceNumber: 1
During the beginning of cell creation,CER is 1