rac sga cache_buffer_size "V$BH"

V$BH:This is a Real Application Clusters view. This view gives the status and number of pings for every buffer in the SGA.
--记录了sga缓冲区中的object及其状态

Column	Datatype	Description
FILE#	NUMBER	Datafile identifier number (to find the filename, query DBA_DATA_FILES or V$DBFILE)
BLOCK#	NUMBER	Block number
CLASS#	NUMBER	Class number
STATUS	VARCHAR2(6)	Status of the buffer: free - Not currently in use xcur - Exclusive scur - Shared current cr - Consistent read read - Being read from disk mrec - In media recovery mode irec - In instance recovery mode
XNC	NUMBER	Number of PCM x to null lock conversions due to contention with another instance. This column is obsolete and maintained for backward compatibility.
LOCK_ELEMENT_ADDR	RAW(4 | 8)	Address of the lock element that contains the PCM lock that is covering the buffer. If more than one buffer has the same address, then these buffers are covered by the same PCM lock.
LOCK_ELEMENT_NAME	NUMBER	The address of the lock element that contains the PCM lock that is covering the buffer. If more than one buffer has the same address, then these buffers are covered by the same PCM lock.
LOCK_ELEMENT_CLASS	NUMBER	The address of the lock element that contains the PCM lock that is covering the buffer. If more than one buffer has the same address, then these buffers are covered by the same PCM lock.
FORCED_READS	NUMBER	Number of times the block had to be reread from the cache because another instance has forced it out of this instance's cache by requesting the lock on the block in exclusive mode
FORCED_WRITES	NUMBER	Number of times GCS had to write this block to cache because this instance had used the block and another instance had requested the lock on the block in a conflicting mode
DIRTY	VARCHAR2(1)	Y - block modified
TEMP	VARCHAR2(1)	Y - temporary block
PING	VARCHAR2(1)	Y - block pinged
STALE	VARCHAR2(1)	Y - block is stale
DIRECT	VARCHAR2(1)	Y - direct block
NEW	VARCHAR2(1)	Always set to N. This column is obsolete and maintained for backward compatibility.
OBJD	NUMBER	Database object number of the block that the buffer represents
TS#	NUMBER	Tablespace number of block

SQL>  select sum(BLOCK#),status from v$bh group by status;

SUM(BLOCK#) STATUS
----------- ---------------------
1.0537E+11 xcur
1.1988E+10 cr

SQL> alter system flush buffer_cache;

System altered.

SQL> select sum(BLOCK#),status from v$bh group by status;

SUM(BLOCK#) STATUS
----------- ---------------------
    854666 xcur
1.1735E+11 free