Although you can allocate additional extents, the blocks themselves are allocated separately. If you allocate an extent to a specific instance, the blocks are immediately allocated to the free list. However, if the extent is not allocated to a specific instance, then the blocks themselves are allocated only when the high water mark moves. The high water mark is the boundary between used and unused space in a segment.

分配extent时,blocks不是马上分配,分成两种情况:

1, 如果extent分配到具体的instance(rac中),blocks马上分配，并归入free list

2, 一般情况下,只有当hwm移动时才分配blocks.

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concepts中关于block分配时机的说明

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