最近开始看concept,很多不能理解的。下面这段话什么意思?是指rac环境下block的分配问题么?求高手指教。
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 moves. The
high water is the boundary between used and unused space in a segment.
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 moves. The
high water is the boundary between used and unused space in a segment.
虽然可以分配扩展,但是扩展中的数据块是被独立分配的。如果将扩展分配给一个特定的实例,扩展中的数据块会立即分配到可用空间链表。但是,如果不是将扩展分配给一个特定的实例,则扩展中的数据块只有到 HWM 移动时才会被分配。 HWM 是段中已用和未用空间的界限。
analyze table tbl compute statistics;
select t.blocks,t.empty_blocks from user_tables t where t.table_name='TBL';
delete from tbl where rownum<10000;
alter table tbl deallocate unused;
analyze table tbl compute statistics;
select t.blocks,t.empty_blocks from user_tables t where t.table_name='TBL';
alter table tbl allocate extent (size 1m instance 1);
analyze table tbl compute statistics;
select t.blocks,t.empty_blocks from user_tables t where t.table_name='TBL';
alter table tbl deallocate unused;
analyze table tbl compute statistics;
select t.blocks,t.empty_blocks from user_tables t where t.table_name='TBL';
http://download.oracle.com/docs/cd/A81042_01/DOC/paraserv.816/a76968/psimpstg.htm#5892讨论的是多实例并行存储
下面是http://www.zw1840.com/oracle/translation/concepts/html/02.Data%20Blocks,%20Extents,%20and%20Segments.htm的翻译。
出处在用户分配一个新的数据扩展(extent)时,其中的数据块(data block)未必被同时分配。如果用户是为某个数据库对象分配数据扩展 ,那么数据块也同时被立即分配并加入可用块列表(free list)中。如果数据扩展并非专为某数据库对象分配,那么数据块只在高水位线(high water )移动时才被分配。高水位线是段(segment)中已用和未用空间的边界。如果按这种理解,什么时候会出现并非为某数据库对象分配区的情况。