Tuning the Cluster Interconnect - Expert Oracle RAC Performance Diagnostics and Tuning - page 482

Figure 14-13. Cluster database performance
The EM cluster cache coherency screen (Figure 14-14 ) is also a tool to monitor cluster interconnect performance.
The figure displays three important matrixes:
Global Cache Block Access Latency : This represents the elapsed time from when the
block request was initiated until it completes. However, when a database block of any class
is unable to locate a buffered copy in the local cache, a global cache operation is initiated
by checking if the block is present in another instance. If it is found, it is shipped to the
requestor.
Global Cache Block Transfer Rate : If a logical read fails to find a copy of the buffer in the
local cache, it attempts to find the buffer in the database cache of a remote instance. If the
block is found, it is shipped to the requestor. The global cache block transfer rate indicates
the number of blocks received.
Block Access Statistics : This indicates the number of blocks read and the number of blocks
transferred between instances in a RAC cluster.