当并发事务的数量超过在线undo segment的数量时undo segment被smon自动创建， 我记得不知道在哪儿看到过说：undo segment的创建会记录在alert中，但是没有发现...还是记错了...
One of the benefits of AUM is the dynamic creation of undo segments as needed. When the number of concurrent transactions exceeds the number of online undo segments, more segments are brought online. If there are offline segments in the current undo tablespace, they are the first ones to be used. If no offline segments exist, new ones are added as long as sufficient space exists within the tablespace’s datafiles.
[@more@]According to the documentation, transactions should be allocated to undo segments according to a least recently committed algorithm. As a transaction commits, the segment is placed in line behind the other segments that do not have current transactions. In testing, the algorithm was not exact, but was very close. Over the space of 500 transactions, less than 10 cycles (using the undo segments in order) did not match the established pattern. One of the benefits of AUM is the dynamic creation of undo segments as needed. When the number of concurrent transactions exceeds the number of online undo segments, more segments are brought online. If there are offline segments in the current undo tablespace, they are the first ones to be used. If no offline segments exist, new ones are added as long as sufficient space exists within the tablespace’s datafiles.来自 “ ITPUB博客 ” ，链接：https://blog.itpub.net/19602/viewspace-1020831/ ，如需转载，请注明出处，否则将追究法律责任。
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