Oracle Database Tips by Donald Burleson

Oracle10g Grid Computing with RAC
Chapter 7 - Cache Fusion and Inter Instance Coordination

Block Transfers Using Cache Fusion - Example

Let's consider another example of how block shipping takes place. Assume that in a 3-node RAC cluster a typical block (of table 'salesman') is brought into instance 3 by a select operation of user C. Initially, the instance acquires SL0 (shared lock with no past image) and the same Block/Lock-element undergoes many conversions, as different users at different instances handle it. The following operations show a clear movement of the blocks among the instances using cache fusion. It also shows the complexity involved. [Figures 7.6 (a) and (b)]

Assumption: Transactions update with commit, but there is no checkpoint until end.

In Stage (1), the data block is read into the buffer of instance 3, and it opens with a SL0 mode (Shared Local without any past image):

select sales_rank from salesman where salesid = 10;

This gives a value of 30. Thus, the data block is protected by a resource in shared mode (S) and its role is local (L). This indicates that the block only exists in the local cache of instance 3.

Figure 7.6 (a) Data Block Shipping using Cache Fusion (Lock Conversions)