Oracle RAC Study之--Cache Fusion

Concept of cache fusion

       Cache Fusion basically is about fusing the memory buffer cache of multiple instance into one single cache. For example if we have 3 instance in a RAC which is using the same datafiles and each instance is having its own memory buffer cache in there own SGA, then cache fusion will make the database behave as if it has a single instance and the total buffer cache is the sum of buffer cache of all the 3 instance. Below figure shows what I mean.

       This behavior is possible because of high speed interconnect existing in the cluster between each instance. Each of instance is connected to other instance using a high-speed interconnect. This makes it possible to share the memory between 2 or more servers. Previously only datafile sharing was possible, now because of interconnect, even the cache memory can be shared.

       But how this helps? Well, for example if we have a data block in one of the instance and its updating the block and other instance needs the same data block then this data block can be copied from holding instance buffer cache to requesting instance buffer cache using this high-speed interconnect. This high speed interconnect is a private connection made just for sending data blocks and more by instances. External users cannot use this connection. It is this interconnect which makes multiple server behave like a cluster. These servers are bind together using this interconnect.

       Moving further, now we know how the cluster is formed and what is the back bone of cluster and what exactly we call “cache fusion”. Next we will see how cache fusion works. But before that we need to discuss few important headings which is very important to understand. 

We will discuss following topics before discussing Cache Fusion

（1）Cache Coherency

（2）Multi-Version consistency model

（3）Resource Co-ordination – Synchronization

（4）Global Cache Service (GCS)

（5）Global Enqueue Service

（6）Global Resource Directory

（7）GCS resource modes and roles

（8）Past Images

（9）Block access modes and buffer states

      I promise this wont be too heavy. Lets look into the overview of these concepts. I wont be going into the details, just sufficient for you to understand cache fusion.

案例分析：

数据库环境：Oracle 10g RAC

[root@aix211 /]#crs_stat -t

1、在Instance 1上执行select查询

2、在Instance 2上做同样的操作

        这个例子说明，Oracle Cache Fusion技术，主要用于保证同一个数据块的版本一致。比如，一个实例内存中的数据块被修改了，其他的Instance就不能再从磁盘上读取这个数据块，必须使用当前的这个数据块，这样才能保证数据块版本的一致性。   

        而对于各个实例都是通过只读方式访问数据块，则可以考虑直接从磁盘上读取一部分数据，在有些时候，这样的效率反而会更高一些。