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1. How MSSM and ASSM work:

MSSM manages the free data blocks in each segment as freelistl chains (the free area in the block is 10% higher than the default pctfree). When the amount of data inserted into an empty data block reaches the block size 1-pctfree, it stops inserting the block, allocates another new data block for insertion, and removes the block from the free list freelist. Therefore, the utilization rate of each block is 1-pctfree, which is for the occurrence of update operation in the future. this block has row migration or row join, increasing the disk Ibano.

When the data within a block is deleted to the value specified by pctused (the default is 40%), the block is suspended on the freelist so that it can accept the insertion of new segments.

If multiple insert occurs in segment at the same time, it will cause freelist contention, so there can be multiple freelist in one segment. The number of freelist determines the number of insert operations in the same segment at the same time. Multiple freelist can belong to different freelist group.

When grouplist=1, the first block is a segment header block, which stores freelist and HWM information, the rest are data blocks, and the first data block is inserted from the second block.

When freelist group > = 2, the freelist is separated from the segment header and exists in a separate block, where each freelist group accounts for one block, and the real data starts from the block after 1 (segment header) + n (freelist group data) block.

ASSM (default) manages free data blocks in segments as bitmap BMB to avoid freelist linked list competition

When the segment space is 1m, the first 16 regions are the same as above, 2 first-level bitmaps manage 1 area from 17 regions, because a first-level bitmap can manage up to 64 blocks.

II. Introduction to ASSM

ASSM is the abbreviation of Automatic Segment Space Management. As opposed to ASSM is Manul Segment Space Management. You can use ASSM,system tablespace only in Locally Managed Tablespace, not ASSM. Locally managed tablespace and ASSm are the default configurations for 11gR2.

In a locally managed tablespace, following parameters are not used any more:

Next

Pctincrease

Minextents

Maxentents

Default

ASSM uses bitmap to manage space, whereas MSSM uses free list to manage free space.

When you create a Tablespace, you can explicitly specify:

Create tablespace tools

Datafile'/ ora01/dbfile/INVREP/tools01.dbf'

Size 100m-Fixed datafile size

Extent management local-Locally managed

Uniform size 128k-Uniform extent size

Segment space management auto-ASSM

Although the tablespace tools uses ASSM, it still needs to grow manually after the space exceeds 100m. Or add datafile. That is, the upper limit of tablespace storage space is determined by the attributes of the data file, not ASSM.

ASSM has many fewer parameters than MSSM, and one Pctfree is sufficient. The following parameters are no longer required

Pctused

Freelist

Freelist group

See how tablespace is managed:

Select tablespace_name, extent_management, segment_space_management

From dba_tablespaces

Order by tablespace_name

III. Assm supplement

Starting with Oracle 9i, there are two ways to manage segment space: manual segment space management and automatic segment space management (Auto Segment Space Management,ASSM)

ASSM uses bitmaps to track or manage each block assigned to an object, and the amount of space left in each block is determined according to the state of the bitmap, such as > 75% Magi 50% mai 75% mai 25% Mel 50% and

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