英文摘自sg，本人理解不够，向大家请教！
When a datafile is placed in backup mode, more redo log entries may be generated because the log writer writes block images of changed blocks of the datafile in backup mode to the redo log instead of just the row information.
为什么当ALTER TABLESPACE users BEGIN BACKUP（online backup）之后,要用images of changed blocks 代替just the row information来写入数据库的，oracle这样做应该是有理由的，但不知道为什么？
Set the datafile or tablespace in backup mode by issuing the ALTER
TABLESPACE...BEGIN BACKUP command. This prevents the sequence number in the datafile header from changing, so that logs are applied in recovery from backup start time. Even if the datafile is in backup mode, it is available for normal transaction.
另外，我理解表空间处于备份状态后直到end backup，它所包含的硬盘上的文件就不会再改变了，这样os cp的文件就是alter ....begin backup时间点后的文件。也就是说开始备份命令执行之后，dbwr不再写data file了。
我理解的对么？
