1、ASSMASSM说明在Orale 9i以前,表的剩余空间的管理与分配都是由链接列表freelist来完成的,因为freelist存在串行的问题因此容易引起往往容易引起段头的争用与空间的浪费(其实这一点并不明显),最主要的还是因为需要DBA花费大量的精力去管理这些争用并监控表的空间利用。自动段空间管理(ASSM),它首次出现在Oracle 920里。有了ASSM,链接列表freelist被位图所取代,它是一个二进制的数组,能够迅速有效地管理存储扩展和剩余区块(free block),因此能够改善分段存储本质,ASSM表空间上创建的段还有另外一个称呼叫Bitmap Managed Segments
2、(BMB段)。让我们看看位图freelist是如何实现的。从使用区段空间管理自动参数创建tablespace开始:create tablespace demodatafile /ora01/oem/demo01.dbf size 5mEXTENT MANAGEMENT LOCAL - Turn on LMTSEGMENT SPACE MANAGEMENT AUTO - Turn on ASSM;一旦你定义好了tablespace,那么表和索引就能够使用各种方法很容易地被移动到新的tablespace里,带有ASSM的本地管理tablespace会略掉任何为PCTUSED、NEXT和FREELI
3、STS所指定的值。当表格或者索引被分配到这个tablespace以后,用于独立对象的PCTUSED的值会被忽略,而Oracle9i会使用位图数组来自动地管理tablespace里表格和索引的freelist。对于在LMT的tablespace内部创建的表格和索引而言,这个NEXT扩展子句是过时的,因为由本地管理的tablespace会管理它们。但是,INITIAL参数仍然是需要的,因为Oracle不可能提前知道初始表格加载的大小。对于ASSM而言,INITIAL最小的值是三个块。新的管理机制用位图来跟踪或管理每个分配到对象的块,每个块有多少剩余空间根据位图的状态来确定,如75%,50%-75%
4、,25%-50%和 alter system dump datafile 7 block 9;System alteredSQL alter system dump datafile 7 block 10;System alteredSQL alter system dump datafile 7 block 11;System altered进入Udump查看刚才生成的trace文件* 2009-12-07 19:30:16.406* SERVICE NAME:(DBA.ANQINGREN.ORG) 2009-12-07 19:30:16.390* SESSION ID:(123.758)
5、2009-12-07 19:30:16.390Start dump data blocks tsn: 8 file#: 7 minblk 9 maxblk 9buffer tsn: 8 rdba: 0x01c00009 (7/9)scn: 0x0000.001a0da0 seq: 0x01 flg: 0x04 tail: 0x0da02001frmt: 0x02 chkval: 0x44e6 type: 0x20=FIRST LEVEL BITMAP BLOCKHex dump of block: st=0, typ_found=1Dump of memory from 0x085C8400
6、to 0x085CA40085C8400 0000A220 01C00009 001A0DA0 04010000 .85C8410 000044E6 00000000 00000000 00000000.D.85C8420 00000000 00000000 00000000 00000000.Repeat 1 times85C8440 00000000 00000000 00000000 00000004.85C8450 FFFFFFFF 0000000D 00000003 00000010.85C8460 00010002 00000000 00000000 00000000.85C847
7、0 00000000 00000003 00000000 00000000.85C8480 00000000 00000000 00000000 00000000.85C8490 01C0000A 00000000 00000000 00000003.85C84A0 00000008 01C0000C 00000000 00000000.85C84B0 00000000 00000000 00000000 00000001.85C84C0 0000D302 00000000 00000000 01C00009.85C84D0 00000008 00000000 01C00011 0000000
8、8.85C84E0 00000008 00000000 00000000 00000000.85C84F0 00000000 00000000 00000000 00000000.Repeat 8 times85C8580 00000000 00000000 00000000 00001011.85C8590 00000000 00000000 00000000 00000000.Repeat 485 times85CA3F0 00000000 00000000 00000000 0DA02001. .Dump of First Level Bitmap Block-nbits : 4 nra
9、nges: 2parent dba:0x01c0000apoffset: 0unformatted: 13total: 16first useful block: 3owning instance : 1instance ownership changed atLast successful SearchFreeness Status:nf1 0nf2 0nf3 0nf4 0Extent Map Block Offset: 4294967295First free datablock : 3Bitmap block lock opcode 0Locker xid:0x0000.000.0000
10、0000Inc #: 0 Objd: 54018HWM Flag: HWM SetHighwater:0x01c0000cext#: 0blk#: 3ext size: 8#blocks in seg. hdrs freelists: 0#blocks below: 0mapblk0x00000000offset: 0-DBA Ranges :-0x01c00009Length: 8Offset: 00x01c00011Length: 8Offset: 80:Metadata1:Metadata2:Metadata3:unformatted4:unformatted5:unformatted6
11、:unformatted7:unformatted8:unformatted9:unformatted10:unformatted11:unformatted12:unformatted13:unformatted14:unformatted15:unformatted-End dump data blocks tsn: 8 file#: 7 minblk 9 maxblk 9* 2009-12-07 19:35:44.296Start dump data blocks tsn: 8 file#: 7 minblk 10 maxblk 10buffer tsn: 8 rdba: 0x01c00
12、00a (7/10)scn: 0x0000.001a0dc1 seq: 0x01 flg: 0x04 tail: 0x0dc12101frmt: 0x02 chkval: 0x5439 type: 0x21=SECOND LEVEL BITMAP BLOCKHex dump of block: st=0, typ_found=1Dump of memory from 0x085C8400 to 0x085CA40085C8400 0000A221 01C0000A 001A0DC1 04010000!.85C8410 00005439 00000000 00000000 000000009T.
13、85C8420 00000000 00000000 00000000 00000000.Repeat 1 times85C8440 00000000 00000000 00000000 01C0000B.85C8450 00000008 00000008 00000000 00000000.85C8460 00000000 00000000 0000D302 00000001.85C8470 00000000 01C00009 00010005 01C00019.85C8480 00010005 01C00029 00010005 01C00039.).9.85C8490 00010005 0
14、1C00049 00010005 01C00059.I.Y.85C84A0 00010005 01C00069 00010005 01C00079.i.y.85C84B0 00010005 00000000 00000000 00000000.85C84C0 00000000 00000000 00000000 00000000.Repeat 498 times85CA3F0 00000000 00000000 00000000 0DC12101.!.Dump of Second Level Bitmap Blocknumber: 8nfree: 8ffree: 0pdba:0x01c0000
15、bInc #: 0 Objd: 54018opcode:0xid:L1 Ranges :-0x01c00009Free: 5 Inst: 10x01c00019Free: 5 Inst: 10x01c00029Free: 5 Inst: 10x01c00039Free: 5 Inst: 10x01c00049Free: 5 Inst: 10x01c00059Free: 5 Inst: 10x01c00069Free: 5 Inst: 10x01c00079Free: 5 Inst: 1-End dump data blocks tsn: 8 file#: 7 minblk 10 maxblk 10Start dump data blocks tsn: 8 file#: 7 minblk 11 maxblk 11buffer tsn: 8 rdba: 0x01c0000b (7/11)scn: 0x0000.001a0dc6 seq:
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