I read from somewhere, I think the wait interface book where it states that the number of clone copies of data block is governed by _DB_BLOCK_MAX_CR_DBA which is set to 6. If there will be more than 6 copies then Oracle will wait for the buffer, this is what I dont get, wait? Can you point us to the original doc? I've never heard that consistent read is blocked and being waited by any reason. (One exception is that buffer lock contention which is caused by physical read - read by other session wait event) As you observed it right, cr blocks are recycled by age. The olest one is replaced by new cr block. You can verify it using x$bh view. Dion Cho PS) "I read somewhere..." kind of thing seems not to be a good habit. You'd better leave a link to original doc or make the replayable test case yourself.