TX enqueue Mode 4的一点补充

在06年的一篇blog TX lock中简略的解释了下TX enqueue的类型和对应的例子。

最近在数据库维护中又有了一点新的发现。

前面文章中提到Mode=4的TX lock又分为下面三种情况

* ITL entry数目不够
* Unique key的约束
* Bitmap index entry

再补充几种情况:

* Index block spilt
* Foreign key enforcement
* Alter tablespace … read only;
* Free Lists slot waits
* Possible with two phase commit
* Extent allocation slow in DMT tablespace

前面几种情况一些文档中多多少少有所提及,index block split较为常见。

最后一种extent allocation slow是实际中遇到的。在DMT的tablespace中,当free extent很多,但不满足分配条件时就会出现extent分配慢的情况,实际中分配一个extent需要8,9秒钟。从而导致大量的session wait在TX-4 enqueue上。

目前还没有找到文档有关于这个的相关说明,也许是因为DMT已经退出历史舞台了吧。

91 thoughts on “TX enqueue Mode 4的一点补充

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