Uwe Hesse wrote:
It is very likely that you experience slow performance with ALTER DATABASE BEGIN BACKUP , because until you do ALTER DATABASE END BACKUP , every modified block is additionally written into the online logfiles .
Doesn't that happen only the first time the block is modified?
It's somewhere in between the two. Specifically, whenever the block is subject to a physical read into the buffer cache, the first change after that results in the whole block being copied into the redo before the change vector is copied.