Which two statements are true concerning buffer states as shown in GV$BH.STATUS in Oracle 12c RAC?
A.
An XCUR block image may exist for a specific database block in only one instance.
B.
A CR image may get served from one instance to another to satisfy a read request.
C.
An XCUR image is not downgraded to a SCUR image for shipping to another instance until commits occur for updated rows on the block.
D.
A PI block image may exist for a specific database block in only one instance.
Explanation:
A: xcur is a RAM block that has been acquired in exclusive current mode. According the Oracle documentation, if a buffer state is exclusive current (XCUR), an
instance owns the resource in exclusive mode.
B: cr mode indicates a “cloned” RAM block (a “stale” block), that was once in xcur mode. The instance has shared access to the block and can only perform reads.
The cr state means the owning instance can perform a consistent read of the block, if the instance holds an older version of the data.
http://www.dba-oracle.com/t_v$bh_status_free_buffer_blocks.htm
A,B
The correct is A & B
C) is wrong because XCUR is downgraded to PI after the block is sent to another instance to be changed and write then.
D) iis wrong because PI is a past image from one block that was sent to another instance and can be transfered to another instance and have the state of PI too, until this new instance write the block to disk by DBWR (after a commit). Then, all PI will be changed to CR.