DBWR Fusion Writes This statistics shows the number of fusion writes. In RAC, as in a single instance Oracle database, blocks are only written to disk for aging, cache replacement, or checkpoints. When a data block is replaced from the cache due to aging or when a checkpoint occurs and the block was changed in another instance but not written to disk, Global Cache Service will request that instance to write the block to disk. Therefore fusion write does not involve an additional write to disk in the first instance. A large number of fusion writes can indicate an ongoing problem. The ratio of fusion writes to the total write request made by the instance is used for performance analysis. The high ratio can indicate the undersized DB cache, or inefficient checkpointing. See also Cache Fusion Writes Ratio for definition of this ratio.
This statistics is exposed in v$sysstat.VALUE 'DBWR fusion writes'