Flash is Flash is Flash, Right?
I was recently catching up on my reading of Jonathan Lewis’ blog when I read his post about Flash Cache. Jonathan’s post contained a few comments on the matter and then refers to Guy Harrison’s post on the topic.
After reading both of those posts I am now convinced more than ever that there is significant confusion over Flash Cache where Oracle Database 11g Release 2 is concerned. Both Jonathan and Guy state that Database Flash Cache (a.k.a. Database Smart Flash Cache) is an Exadata-only feature liberated for use in non-Exadata environments. I need to point out that this is not correct. However, when extremely smart folks are getting it wrong I think it is safe to say that there must not be enough information available.
From the initial release of Oracle Database 11g Release 2 the Database Flash Cache feature has had nothing in common with Exadata Smart Flash Cache other than Flash technology happens to be at the very center. In fact, it would be quite difficult for them to be any more dissimilar than they are. Put simply:
- Database Flash Cache is an extension of the SGA that resides in Flash. Only data buffered in the SGA can make its way into the Database Flash Cache. The data is essentially “aged” out and written by DBWR into the Flash Cache device file.
- Exadata Smart Flash Cache is PCI Flash in Exadata Storage Server used for intelligent, adaptive cache. That is to say that regardless of whether the primary buffering of data is in the SGA or PGA, Exadata Smart Flash Cache is down-wind caching data based upon demand.
The words of the Oracle Database 11g New Features list introduces Database Flash Cache as follows:
Database Smart Flash Cache
Database Smart Flash Cache is an optional memory component that you can add if your database is running on Solaris or Oracle Enterprise Linux. It is an extension of the SGA-resident buffer cache, providing a level 2 cache for database blocks. It can improve response time and overall throughput.
As for Exadata Smart Flash Cache, there is a good white paper on the topic available here.
Part II can be found here.