Oracle Freelist和HWM原理及性能优化

核心提示： segment header中的最大free list个数:Block Size Max # Freelists--- -2K 244K 508K 10116k 204事务T1释放出来的空闲块(DELETE or UPDATE)的使用 :l 立即被T1所重用l 当T1 commit后被其它

segment header中的最大free list个数:

Block Size Max # Freelists

----------- -----------------

2K 24

4K 50

8K 101

16k 204

事务T1释放出来的空闲块(DELETE or UPDATE)的使用 :

l 立即被T1所重用

l 当T1 commit后被其它需要空闲块的事务重用，过程举例如下:

2.5 HMW概念

HIGH WATER MARK代表一个表使用的最大的(top limit)块 。2.1中已经提到HIGH WATER MARK 记录在segment header中，并且在Oracle插入数据时一般增长5个blocks(并非总是5个块，具体参见2.4.2中流程图中HMW增长方式)。

　　segment header block中与HWM相关信息说明如下:　　 EXTENT CONTROL:
　　Extent Header:: spare1: 0 space2: 0 #extents: 13 #blocks: 1429
　　last map 0x00000000 #maps: 0 offset: 4128
　　Highwater:: 0x020004d0 ext#: 12 blk#: 275 ext size: 475
　　#blocks in seg. hdr’s freelists: 5
　　#blocks below: 1229
　　mapblk 0x00000000 offset: 12
　　Unlocked
　　==> spare1: this field is no longer used (old inc#, now always 0)
　　==> space2: this field is no longer used (old ts#, now always 0)
　　==> #extents: number of extents allocated to segment
　　==> #blocks: number of blocks allocated to segment
　　==> last map: address of last extent map block
　　0 if extent map is entirely in the segment header
　　==> #maps: number of extent map block
　　==> offset: offset to end of extent map
　　==> HWM dba: address of block at highwater mark
　　==> ext#: HWM extent number relative to segment
　　==> blk#: HWM block number within extent
　　==> ext size: HWM extent size (in blocks)
　　==> #blocks in seg. hdr’s freelists: number of blocks in seg. hdr’s free list
　　==> #blocks below: number of blocks below HWM
　　==> mapblk dba: dba of extent map block containing HWM extent
　　is 0 if HWM is in the segment header
　　==> offset: offset within extent map block
　　is the ext# if HWM is in segment header
　　==> Locked by: if locked by a transaction, the xid is displayed