关于dirty buffer

其实大家对v$bh这个视图可能一直重视不够.

我们来看一下这个视图主要字段说明:

• 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

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我们注意到v$dirty代表的就是block modified

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如果我们进一步向下追溯:

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 (+)

我们可以看到v$bh的底层表示x$bh

其中v$bh中的dirty/temp/ping/stale/direct五个字段来源于x$bh中的flag字段

格式化一下输出:

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(+)

由此我们得以窥视x$bh.flag的深层含义.