Oracle RAC Cache Fusion Reads and Writes

Cache Fusion reads occur when a user on one system queries a block and then a user on another system queries the same block. The block is passed over the high speed interconnect (versus being read from disk). Cache fusion writes occur when a block previously changed by another instance needs to be written to disk in response to a checkpoint or cache aging. When this occurs, Oracle sends a message to notify the other instance that a fusion write will be performed to move the data block to disk. Fusion writes do not require an additional write to disk and are a subset of all physical writes incurred by an instance. The ratio DBWR fusion writes to physical writes shows the proportion of writes that Oracle manages with fusion writes.

Here is a query to determine ratio of cache fusion writes:

 

 

Here is some sample output:

 

 

 

A larger than usual value for cache fusion writes ratio may indicate:

• Insufficiently large caches
• Insufficient checkpoints
• Large numbers of buffers written due to cache replacement or checkpointing