一. 官网说明
Logical Storage Structures
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.
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 with automatic segment space management (ASSM) or manual segment space management (MSSM).
· Dictionary-managed tablespaces
The database uses the data dictionary to manage extents (see "Overview of the Data Dictionary").
Figure 12-3 shows 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 tablespace maintains a bitmap in the data file header to track free and used space in the data file body. Each bit corresponds to a group 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. 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.
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
Multiple transactions can 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 instan