Reclaimable storage 可回收数据存储

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####DB2 数据库认证考试614的题目：####
   **No2.What is the preferred method for reclaiming unused storage in a DMS table space?**
A. Perform an ALTER TABLESPACE...REDUCE operation.
B. Perform a REORG on each table within the table space, then perform an ALTER
TABLESPACE...REDUCE operation.
C. Perform an ALTER TABLESPACE...LOWER HIGH WATER MARK operation, and then
an ALTER TABLESPACE...REDUCE operation.
D. Perform a REORG on each table within the table space, then perform an ALTER
TABLESPACE...LOWER HIGH WATER MARK operation.

**提干问对于DMS表空间下面四种方案哪种是回收可用存储的首选方案？**

***首先我们先来聊一聊回收存储的(Reclaimable storage) 的概念。***

Reclaimable storage是DB2®v9.7及更高版本的非临时自动存储和DMS表空间所具有的特征。您可以使用它来整合高水位标记以下在使用的区块，并将你表空间中未使用的区段返回给系统以供重用。 
	对于DB2V9.7以前版本中创建的表空间来说，系统释放存储的唯一方法drop容器，或通过释放高水位线以上未使用的区块来减小容器的SIZE。没有高水位下降的直接机制。可以通过卸载和重新加载数据到一个空的表空间降低高水位线，或通过间接操作像执行表和索引的重组。对于后一种方法而言，高水位线可能仍然不能降低，即使在High Water Mark下面有可用的区块。
	在区块整合过程中，包含数据的区域被移动到高水位以下的未使用的区域。在移动区块之后，如果在高水位下仍有free区块，则这些free的区块就会被释放为可用空闲区块。紧接着，高水位线会移动到紧邻最后被使用的区段的页上。在存储可回收的表空间中，可以使用ALTER TABLESPACE语句回收重组区块。图1演示了如何回收存储的过程。
    回收存储的运行原理：当一个表空间属于可回收存储类型时，上层区块上的数据会移动到下层的区块上，从而可以释放上层的区块。(图1)
    DB2 V9.7和更高级的版本中所有非临时自动存储和DMS表空间都提供了对高水位下区块的整合功能。对于V9.7之前版本中的表空间，你必须首先在V9.7版本中创建一个新的DB2表空间用以取代之前版本的表空间。你可以卸载和重新加载数据或使用sysproc.admin_move_table程序在线表级移动数据。但是,这种迁移不是必需的。同一个数据库中可回收存储的表空间和存储不可回收的表空间是可以共存的。
    通过移动区块减少表空间的大小属于联机操作。换句话说，当我们在执行上述操作时，数据操作语言（DML）和数据定义语言（DDL）可以同步运行。但是，一些操作如备份或还原，不能与移动区块的同时运行。在这些情况下，需要访问被移动的区段的操作（例如，备份）会一直处于等待状态，直到移动了多个区段（这个数字是非用户可配置的）后备份进程获得有关所在区段的锁，备份进程才会继续进行。
    你可以使用mon_get_extent_movement_status函数监控区块移动的进展程度。 
图1
![图1]( https://www.ibm.com/support/knowledgecenter/SSEPGG_10.1.0/com.ibm.db2.luw.admin.dbobj.doc/doc/00055395.gif)

Tip:
    ***为了通过使用ALTER TABLESPACE 操作最大限度的回收表空间，首先需要对表空间中的tables和indexes执行REORG操作；***

###自动存储表空间###

####可以通过多种方式减少自动存储表空间：####

1).Container reduction only;
    2).Lower high water mark only;
    3).Lower high water mark and reduce containers by a specific amount;
    4).Lower high water mark and reduce containers the maximum amount possible;

###DMS表空间###
####有两种方式可以用于减小DMS表空间:####
    1).Container reduction only
    2).Lower high water mark only
   Lowering the high water mark and reducing container size is a combined, automatic operation with automatic storage table spaces. By contrast, with DMS table spaces, to achieve both a lowered high water mark and smaller container sizes, you must perform two operations:
    First, you must lower the high water mark for the table space using the ALTER TABLESPACE statement with the LOWER HIGH WATER MARK clause.
    Next you must use the ALTER TABLESPACE statement with the REDUCE database-container clause by itself to perform the container resizing operations.
 
####示例####

Example 1: Reducing the size of an automatic storage table space by the maximum amount.
Assuming a database with one automatic storage table space TS and three tables T1, T2, and T3 exists, we drop tables T1 and T3:

DROP TABLE T1 
   DROP TABLE T3

Now, assuming that the extents are now free, the following statement causes the extents formerly occupied by T1 and T3 to be reclaimed, and the high water mark of the table space reduced:

ALTER TABLESPACE TS REDUCE MAX

Example 2: Reducing the size of an automatic storage table space by a specific amount.
Assume that we have a database with one automatic storage table space TS and two tables T1, and T2. Next, we drop table T1:

DROP TABLE T1

Now, to reduce the size of the table space by 1 MB, use the following statement:

ALTER TABLESPACE TS REDUCE SIZE 1M

Alternatively, you could reduce the table space by a percentage of its existing size with a statement such as this:

ALTER TABLESPACE TS REDUCE SIZE 5 PERCENT

Example 3: Reducing the size of an automatic storage table space when there is free space below the high water mark.
Like Example 1, assume that we have a database with one automatic storage table space TS and three tables T1, T2, and T3. This time, when we drop T2 and T3, there is a set of five free extents just below the high water mark. Now, assuming that each extent in this case was made up of two 4K pages, there is actually 40 KB of free space just below the high water mark. If you issue a statement such as this one:

ALTER TABLESPACE TS REDUCE SIZE 32K

the database manager can lower the high water mark and reduce the container size without the need to perform any extent movement. This scenario is illustrated in Figure 2
Figure 2. Lowering the high water mark without needing to move extents.
![图2]( https://www.ibm.com/support/knowledgecenter/SSEPGG_10.1.0/com.ibm.db2.luw.admin.dbobj.doc/doc/00055404.gif )
Example 4: Reducing the size of a DMS table space.
Assume that we have a database with one DMS table space TS and three tables T1, T2, and T3. Next, we drop tables T1 and T3:

DROP TABLE T1 
   DROP TABLE T3

To lower the high water mark and reduce the container size with DMS table space is a two-step operation. First, lower the high water mark through extent movement with the following statement:

ALTER TABLESPACE TS LOWER HIGH WATER MARK

Next, you would reduce the size of the containers with a statement such as this one:

ALTER TABLESPACE TS REDUCE (ALL CONTAINERS 5 M)
  
  所以本题实际上只是针对DMS 表空间的理论考察，就像文中讲到的“with DMS table spaces, to achieve both a lowered high water mark and smaller container sizes, you must perform two operations:”
    First, you must lower the high water mark for the table space using the ALTER TABLESPACE statement with the LOWER HIGH WATER MARK clause.
    Next you must use the ALTER TABLESPACE statement with the REDUCE database-container clause by itself to perform the container resizing operations.

Reclaimable storage Db2

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