How Exadata Offload Some RMAN Incremtal Backup Load To Cell

It's well known that from 10g, oracle db has a new feature to enable block change tracking. It does a great job to avoid RMAN to scan the whole data files while only track the data that have been changed since the last backup.

At that time, exadata was not out yet. With new Exadata with storage cell storage, It enhances the block change tracking and offload some incremental backup load to Cell. Here is how it works

The key is on _bct_chunk_size which is hidden parameter ,default is 0 .
However CTWR(Change Tracking Writer) uses 32K(a chunk).It can't be 0 though in reality.
If we have DB Block set 8K, it means the minimum  track unit of CTWR is 4 db blocks.
If one of 4 is changed, all 4 are tracked in tracking files

Before Exadata, RMAN need some work to pick the changes blocks from the chunk. Now with Exadata, this work can be offloaded into Cell. On Exadata, oracle recommend AU size to be 4M.
4M can hold quite a few 32k(chunk) . It gives enough space for Cell to filter or inspect blocks in the AU, then return the only changed blocks to Database, thus offload some load for RMAN.

To check ASM AU size is simple:
select ALLOCATION_UNIT_SIZE,NAME from v$asm_diskgroup;