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extent的概念

原创 Linux操作系统 作者:oracle_ace 时间:2008-01-08 13:32:03 0 删除 编辑

有的时候读一读concept陶冶一下情操,还是有益处的,平时和Oracle打交道打多了,偶尔我们都会忽略一些最最基本的概念。做为技术人员,需要有一种知其然而且要知其所以然的精神,在我们不断往前走的时候,还是应该停下来休息一会多巩固一下基础,基础真的很重要。

An extent is a specific number of contiguous data blocks allocated for storing a specific type of information.
(可以知道的是一个区中的blocks都是相邻的)

The level of logical database storage above an extent is called a segment. A segment is a set of extents, each of which has been allocated for a specific data structure and all of which are stored in the same tablespace. For example, each table's data is stored in its own data segment, while each index's data is stored in its own index segment. If the table or index is partitioned, each partition is stored in its own segment.
(一个段当然是一系列的extent组成,并且都是存在于一个表空间中)

Oracle allocates space for segments in units of one extent. When the existing extents of a segment are full, Oracle allocates another extent for that segment. Because extents are allocated as needed, the extents of a segment may or may not be contiguous on disk.
(段分配空间的单元就是extent,因为extent是按需分配,因此段中的extent在磁盘上不可能也可能是连续的)

A segment and all its extents are stored in one tablespace. Within a tablespace, a segment can include extents from more than one file; that is, the segment can span datafiles. However, each extent can contain data from only one datafile.
(段和extent都一定是存储在一个表空间中,而一个表空间由多个数据文件组成,而且段中所分配的extent是可以来自表空间中不同的数据文件,因此段是可以跨越数据文件的,而extent只能在一个数据文件中,不能跨越数据文件)

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.

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