关于回滚段的探讨（推出第一篇文章）

shangym

不客气，呵呵
biti是不是又要卸了再装了啊

gaobo

不错，BITI就是BITI，希望大家能从中挑出些毛病来。

jgwang

biti,

Can you zip pdf and upload again, if possible? I always have problem with downloading attachment in pdf.

I am using English version of Win2000 and IE. Does that matter?

hqs4500

装了acrobat reader5.1就看见了。
几个问题：
1、“在初始化参数文件中有一个参数transactions_per_rollback_segment，决定了同一个回滚段中所允许最大的同时存在的最大事务数“，据我所知，该参数没有这样的作用。CONCEPTS 中原文如下：
The TRANSACTIONS_PER_ROLLBACK_SEGMENT parameter does not limit the number of transactions that can use a rollback segment. Rather, it determines the number of rollback segments an instance attempts to acquire when opening a database
2、文中关于参数rollback_segments与回滚段类型PUBLIC、PRIVATE的关系的论断，似乎不太对。正确的描述如下：实例启动时，oracle自动online的回滚段个数受控于：n=ceil(transactions/transactions_per_rollback_segment)，当rollback_segments指定的回滚段个数m小于n，系统自动online public的回滚段，直到个数等于n，如果还有另外一些回滚段，不管是PUBLIC或PRIVATE，都将保持offline的状态。小结之：
a.在rollback_segments指定的，不管是PUBLIC或PRIVATE，都将自动online
b.不在rollback_segments指定的，只有PUBLIC才可能自动online
c.不在rollback_segments指定的PRIVATE的回滚段可以用alert使之online
3、关于回缩时机、如何回缩没有讲得很清楚，但我自己也不能把它讲清楚
请大家指正。

piner

联机手册上是这么写的
[B]TRANSACTIONS_PER_ROLLBACK_SEGMENT specifies the number of concurrent transactions allowed per rollback segment. [/B]The minimum number of rollback segments acquired at startup is TRANSACTIONS divided by the value for this parameter. For example, if TRANSACTIONS is 101 and this parameter is 10, then the minimum number of rollback segments acquired would be the ratio 101/10, rounded up to 11.

More rollback segments can be acquired if they are named in the parameter ROLLBACK_SEGMENTS.

piner

第二点的确是这样的，你的是正确的
publict回滚段可以自动启动，但是不一定自动启动，
主要是看是否超过数据库默认启动的个数
当然，如果加入ROLLBACK_SEGMENTS就肯定启动了

biti_rainy

已修正

关于 回缩时机 ,请再看看是否觉得清楚？

hqs4500

最初由 piner 发布
[B]联机手册上是这么写的
[B]TRANSACTIONS_PER_ROLLBACK_SEGMENT specifies the number of concurrent transactions allowed per rollback segment. [/B]The minimum number of rollback segments acquired at startup is TRANSACTIONS divided by the value for this parameter. For example, if TRANSACTIONS is 101 and this parameter is 10, then the minimum number of rollback segments acquired would be the ratio 101/10, rounded up to 11.

More rollback segments can be acquired if they are named in the parameter ROLLBACK_SEGMENTS. [/B]

你引的这段在reference中，而我引的在concepts中，似乎相互矛盾。到底哪个是正确的？

hqs4500

http://www.ixora.com.au/q+a/undo.htm说：
Incidentally, setting transactions_per_rollback_segment to 1 does not prevent rollback segment sharing. It only governs the number of public rollback segment to attempt to acquire at instance startup.
看来，concepts种说法是对的。hehe。

biti_rainy

TRANSACTIONS_PER_ROLLBACK_SEGMENT specifies the number of concurrent transactions you expect each rollback segment to have to handle. The minimum number of rollback segments acquired at startup is TRANSACTIONS divided by the value for this parameter. For example, if TRANSACTIONS is 101 and this parameter is 10, then the minimum number of rollback segments acquired would be the ratio 101/10, rounded up to 11.

而piner 摘的是 allowed

concepts 中跟 hqs4500 的是一致的

看来是编写文档的人的问题！
  

实际上，通常这对我们没有构成影响