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

一. 官网说明

Logical Storage Structures

Oracle.com/docs/cd/E11882_01/server.112/e16508/logical.htm#CNCPT7668

其中有关ASSM 的内容如下：

Oracle Database must use logical space management to 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.

· Locally managed tablespaces (default)

.

· Dictionary-managed tablespaces

shows the alternatives for logical space management in a tablespace.

The following graphic is a conceptual representation of bitmap-managed storage. A 1 in the header refers to used space, whereas a 0 refers 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 management is 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 tablespace users can be managed automatically while segments in tablespace tools are managed manually.

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

· Simplified administration

).

· Increased concurrency

· 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.

(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 to PCTFREE, MSSM requires you to control space allocation with SQL parameters such as PCTUSED, FREELISTS, and FREELIST GROUPS. PCTUSED sets 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 set PCTUSED to 40 in a CREATE TABLE statement, 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 to PCTUSED, 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 than PCTUSED. 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.

, which are collections of free lists. Each group has a master free list that manages the individual process free lists in the group. Space overhead for free lists, especially for free list groups, can be significant.

PCTUSED is 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, PCTFREE determines whether a new row can be inserted into a block, but it does not use free lists and ignores PCTUSED.