Oracle 表空间管理（tablespace managment）

Logical Storage Structures

其中有关ASSM 的内容如下：

Oracle Database must uselogical space managementto track and allocate the extents in a tablespace. When a database object requires an extent, the database must have a method of finding and providing it. Similarly, when an object no longer requires an extent, the database must have a method of making the free extent available.

Oracle Database manages space within a tablespace based on the type that you create. You can create either of the following types of tablespaces:

Locally managed tablespaces (default)

The database uses bitmaps in the tablespaces themselves to manage extents. Thus, locally managed tablespaces have a part of the tablespace set aside for a bitmap. Within a tablespace, the database can manage segments withautomatic segment space management (ASSM)ormanual segment space management (MSSM).

Dictionary-managed tablespaces

The database uses thedata dictionaryto manage extents (see“Overview of the Data Dictionary”).

Figure 12-3shows the alternatives for logical space management in a tablespace.

Figure 12-3 Logical Space Management

Description of “Figure 12-3 Logical Space Management”

A locally managed tablespacemaintains a bitmap in the data file header to track free and used space in the data file body. Each bit corresponds to a g免费云主机域名roup of blocks. When space is allocated or freed, Oracle Database changes the bitmap values to reflect the new status of the blocks.

The following graphic is a conceptual representation of bitmap-managed storage. A1in the header refers to used space, whereas a0refers to free space.

A locally managed tablespace has the following advantages:

Avoids using the data dictionary to manage extents

Recursive operations can occur in dictionary-managed tablespaces if consuming or releasing space in an extent results in another operation that consumes or releases space in a data dictionary table or undo segment.

Tracks adjacent free space automatically

In this way, the database eliminates the need to coalesce free extents.

Determines the size of locally managed extents automatically

Alternatively, all extents can have the same size in a locally managed tablespace and override object storage options.

Note:

Oracle strongly recommends the use of locally managed tablespaces with Automatic Segment Space Management.

Segment space managementis an attribute inherited from the tablespace that contains the segment. Within a locally managed tablespace, the database can manage segments automatically or manually. For example, segments in tablespaceuserscan be managed automatically while segments in tablespacetoolsare managed manually.

Automatic Segment Space Management

The ASSM method uses bitmaps to manage space. Bitmaps provide the following advantages:

Simplified administration

ASSM avoids the need to manually determine correct settings for many storage parameters.Only one crucial SQL parameter controls space allocation:PCTFREE.This parameter specifies the percentage of space to be reserved in a block for future updates (see“Percentage of Free Space in Data Blocks”).

ASSM管理，只需要一个参数：PCTFREE

Increased concurrency

Multipletransactionscan search separate lists of free data blocks, thereby reducing contention and waits. For many standard workloads, application performance with ASSM is better than the performance of a well-tuned application that uses MSSM.

Dynamic affinity of space to instances in an Oracle Real Application Clusters (Oracle RAC) environment

ASSM is more efficient and is the default for permanent, locally managed tablespaces.

Note:

This chapter assumes the use of ASSM in all of its discussions of logical storage space.

Manual Segment Space Management

The legacy MSSM methoduses a linked list called afree listto manage free space in the segment. For a database object that has free space, a free list keeps track of blocks under thehigh water mark(HWM), which is the dividing line between segment space that is used and not yet used. As blocks are used, the database puts blocks on or removes blocks from the free list as needed.

In addition toPCTFREE, MSSM requires you to control space allocation with SQL parameters such asPCTUSED,FREELISTS, andFREELIST GROUPS.PCTUSEDsets the percentage of free space that must exist in a currently used block for the database to put it on the free list. For example, if you setPCTUSEDto40in aCREATE TABLEstatement, then you cannot insert rows into a block in the segment until less than 40% of the block space is used.

As an illustration, suppose you insert a row into a table. The database checks a free list of the table for the first available block. If the row cannot fit in the block, and if the used space in the block is greater than or equal toPCTUSED, then the database takes the block off the list and searches for another block. If you delete rows from the block, then the database checks whether used space in the block is now less thanPCTUSED. If so, then the database places the block at the beginning of the free list.

An object may have multiple free lists. In this way, multiple sessions performing DML on a table can use different lists, which can reduce contention. Each database session uses only one free list for the duration of its session.

As shown inFigure 12-4, you can also create an object with one or morefree list groups, which are collections of free lists. Each group has amaster free listthat manages the individualprocess free listsin the group. Space overhead for free lists, especially for free list groups, can be significant.

Figure 12-4 Free List Groups

Description of “Figure 12-4 Free List Groups”

Managing segment space manually can be complex. You must adjustPCTFREEandPCTUSEDto reduce row migration (see“Chained and Migrated Rows”) and avoid wasting space. For example, if every used block in a segment is half full, and ifPCTUSEDis 40, then the database does not permit inserts into any of these blocks. Because of the difficulty of fine-tuning space allocation parameters, Oracle strongly recommends ASSM.In ASSM,PCTFREEdetermines whether a new row can be inserted into a block, but it does not use free lists and ignoresPCTUSED.

— ASSM会忽略PCTUSED参数

Dictionary-Managed Tablespaces

A dictionary-managed tablespace uses the data dictionary to manage its extents.Oracle Database updates tables in the data dictionary whenever an extent is allocated or freed for reuse.For example, when a table needs an extent, the database queries the data dictionary tables, and searches for free extents. If the database finds space, then it modifies one data dictionary table and inserts a row into another. In this way, the database manages space by modifying and moving data.

The SQL that the database executes in the background to obtain space for database objects isrecursive SQL.Frequent use of recursive SQL can have a negative impact on performance because updates to the data dictionary must be serialized.Locally managed tablespaces, which are the default, avoid this performance problem.

二. ASSM说明

在Orale 9i以前，表的剩余空间的管理与分配都是由链接列表freelist来完成的，因为freelist存在串行的问题因此容易引起往往容易引起段头的争用与空间的浪费（其实这一点并不明显），最主要的还是因为需要DBA花费大量的精力去管理这些争用并监控表的空间利用。
自动段空间管理（ASSM），它首次出现在Oracle 920里。有了ASSM，链接列表freelist被位图所取代，它是一个二进制的数组，能够迅速有效地管理存储扩展和剩余区块（free block），因此能够改善分段存储本质，ASSM表空间上创建的段还有另外一个称呼叫Bitmap Managed Segments（BMB段）。

让我们看看位图freelist是如何实现的。从使用区段空间管理自动参数创建tablespace开始：

create tablespace demo

datafile ‘/ora01/oem/demo01.dbf ‘

size 5m

EXTENT MANAGEMENT LOCAL — Turn on LMT

SEGMENT SPACE MANAGEMENT AUTO — Turn on ASSM;

一旦你定义好了tablespace，那么表和索引就能够使用各种方法很容易地被移动到新的tablespace里，带有ASSM的本地管理tablespace会略掉任何为PCTUSED、NEXT和FREELISTS所指定的值。

当表格或者索引被分配到这个tablespace以后，用于独立对象的PCTUSED的值会被忽略，而Oracle9i会使用位图数组来自动地管理tablespace里表格和索引的freelist。对于在LMT的tablespace内部创建的表格和索引而言，这个NEXT扩展子句是过时的，因为由本地管理的tablespace会管理它们。但是，INITIAL参数仍然是需要的，因为Oracle不可能提前知道初始表格加载的大小。对于ASSM而言，INITIAL最小的值是三个块。

新的管理机制用位图来跟踪或管理每个分配到对象的块，每个块有多少剩余空间根据位图的状态来确定，如>75%,50%-75%,25%-50%和<25%，也就是说位图其实采用了四个状态位来代替以前的pctused，什么时候该利用该数据块则由设定的pctfree来确定。

使用ASSM的一个巨大优势是，位图freelist肯定能够减轻缓冲区忙等待（buffer busy wait）的负担，这个问题在Oracle9i以前的版本里曾是一个严重的问题。

在没有多个freelist的时候，每个Oracle表格和索引在表格的头部都曾有一个数据块，用来管理对象所使用的剩余区块，并为任何SQL插入声明所创建的新数据行提供数据块。当数据缓冲内的数据块由于被另一个DML事务处理锁定而无法使用的时候，缓冲区忙等待就会发生。当你需要将多个任务插入到同一个表格里的时候，这些任务就被强制等待，而同时Oracle会在同时分派剩余的区块，一次一个。

有了ASSM之后，Oracle宣称显著地提高了DML并发操作的性能，因为（同一个）位图的不同部分可以被同时使用，这样就消除了寻找剩余空间的串行化。根据Oracle的测试结果，使用位图freelist会消除所有分段头部（对资源）的争夺，还能获得超快的并发插入操作

尽管ASSM显示出了令人激动的特性并能够简化Oracle DBA的工作，但是Oracle9i的位图分段管理还是有一些局限性的：

1.一旦DBA被分配之后，它就无法控制tablespace内部的独立表格和索引的存储行为。

2.大型对象不能够使用ASSM，而且必须为包含有LOB数据类型的表格创建分离的tablespace。

3.你不能够使用ASSM创建临时的tablespace。这是由排序时临时分段的短暂特性所决定的。

4.只有本地管理的tablespace才能够使用位图分段管理。

5使用超高容量的DML（例如INSERT、UPDATE和DELETE等）的时候可能会出现性能上的问题。

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