可是在ORACLE文档《DB concepts》中写到：
A tablespace that manages its own extents maintains a bitmap in each datafile to
keep track of the free or used status of blocks in that datafile. Each bit in the bitmap
corresponds to a block or a group of blocks.
那么什么时候是a group of blocks？
另外不清楚你说的2level bitmap,能否解释一下？
