I will explain Smart Flash cache feature of Oracle Exadata in this article.
Read previous article before this.
Exadata supports two types of flash cache mode: Write Through and Write Back. Write Through is default mode of Exadata.
- In this mode, when the write request, CellSRV writes the data to disk and sends confirmation to the database to continue the other operations and expects to receive an appropriate call to learn whether the data will be cache or not. Therefore, when this mode is used, no advantage in writing mode is achieved, but the sequential read operation is advantageous due to cache.
- If the data will be read for the first time when the read request, CellSRV will decide whether to cache or not by considering various factors.
Exadata Storage Software 184.108.40.206.0 version in the Write back mode with write-intensive environments is selected to be advantageous when selected.
Exadata Storage Software 220.127.116.11.0 version with write-intensive environments in the Write back mode is selected to be advantageous when selected. When this mode is activated, the following actions are taken when the write operation is performed.
Write back mode in the Exadata Storage Software 18.104.22.168.0 have been more advantageous in the write-intensive databases. When this mode is activated, the following actions are done when the write operation is performed.
- When the write request, CellSRV uses an intelligent Cache algorithm whether cache this request or not.
- If the answer is OK, that data cached and and a confirmation will be sent to the database.
- Data kept in cache can only be written to disk when it is used too little or when it is old.
You can learn which mode is used in Storage Server with the following query.
Or you can learn it from detailed features of Cell as follows.
Yada Cell in detaylı özelliklerinden de aşağıdaki gibi görebilirsiniz.
The following steps can be used to activate Writeback mode in Rolling mode.
CellCLI> drop flashcache CellCLI> list griddisk attributes name,asmmodestatus,asmdeactivationoutcome CellCLI> alter griddisk all inactive CellCLI> alter cell shutdown services cellsrv CellCLI> alter cell flashCacheMode=WriteBack CellCLI> alter cell startup services cellsrv CellCLI> alter griddisk all active CellCLI> create flashcache all CellCLI> list cell detail [OR] list cell attributes flashCacheMode
We need to perform above steps on each Cell Server. We can also do this in non-Rolling mode. However, when done in non-rolling mode, we need to have full downtime.
Writeback mode provides serious advantage to write to Cache/memory instead of disk. This mode is very suitable in database where writing operations are more than read requests. In general Write back is suitable for OLTP database, and Write Through is perfect for OLAP environments.