關於dirty buffer

其實大家對v$bh這個視圖可能一直重視不夠.

我們來看一下這個視圖主要欄位說明:


STATUS

VARCHAR2(1)

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 ?


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


?

我們注意到v$dirty代表的就是block modified

?

如果我們進一步向下追溯:

SQL> select VIEW_DEFINITION from v$fixed_view_definition where VIEW_NAME = 'GV$BH';

VIEW_DEFINITION
--------------------------------------------------------------------------------
select bh.inst_id, file#, dbablk, class, decode(state,0,'free',1,'xcur',2,'scur'
,3,'cr', 4,'read',5,'mrec',6,'irec',7,'write',8,'pi'), x_to_null, forced_reads,
forced_writes, bh.le_addr, name,le_class, decode(bitand(flag,1), 0, 'N', 'Y'), d
ecode(bitand(flag,16), 0, 'N', 'Y'), decode(bitand(flag,1536), 0, 'N', 'Y'), dec
ode(bitand(flag,16384), 0, 'N', 'Y'), decode(bitand(flag,65536), 0, 'N', 'Y'), '
N', obj, ts# from x$bh bh, x$le le where bh.le_addr = le.le_addr (+)


我們可以看到v$bh的底層表示x$bh

其中v$bh中的dirty/temp/ping/stale/direct五個欄位來源於x$bh中的flag欄位

格式化一下輸出:

SELECT bh.inst_id, file#, dbablk, CLASS,
?????? DECODE (state,
?????????????? 0, 'free',
?????????????? 1, 'xcur',
?????????????? 2, 'scur',
?????????????? 3, 'cr',
?????????????? 4, 'read',
?????????????? 5, 'mrec',
?????????????? 6, 'irec',
?????????????? 7, 'write',
?????????????? 8, 'pi'
????????????? ),
?????? x_to_null, forced_reads, forced_writes, bh.le_addr, NAME, le_class,
?????? DECODE (BITAND (flag, 1), 0, 'N', 'Y'),
?????? DECODE (BITAND (flag, 16), 0, 'N', 'Y'),
?????? DECODE (BITAND (flag, 1536), 0, 'N', 'Y'),
?????? DECODE (BITAND (flag, 16384), 0, 'N', 'Y'),
?????? DECODE (BITAND (flag, 65536), 0, 'N', 'Y'), 'N', obj, ts#
? FROM x$bh bh, x$le le
?WHERE bh.le_addr = le.le_addr(+)


由此我們得以窺視x$bh.flag的深層含義.