AUTOMATIC SEGMENT SPACE MANAGEMENT
Prior to Oracle 9i, each block had to be read so the freelist could be checked to see if there was room in the block. In 9i, the bitmap can be checked reducing the number of blocks read unnecessarily.
Starting from Oracle9i, the PCTUSED, FREELISTS, and FREELIST GROUPS parameters are ignored with locally managed tablespaces and ASSM.The ASSM tablespace is implemented by adding the SEGMENT SPACE MANAGEMENT AUTO clause to the tablespace definition syntax. ASSM tablespaces automate freelist management by replacing the traditional one-way linked-list freelists with bitmap freelists, and remove the ability to specify PCTUSED, FREELISTS, and FREELIST GROUPS storage parameters for individual tables and indexes.
In an Locally Managed Tablespace, Oracle moves the tablespace information out of the data dictionary tablespace and stores it directly within the tablespace itself. The second major tablespace enhancement in Oracle9i, was automatic segment space management (ASSM). a.k.a. bitmap freelists. With ASSM, the linked-list freelists are replaced with bitmaps, a binary array that turns out to be very fast and efficient for managing storage extents and free blocks, thereby improving segment storage internals.
Automatic free space management is only available in locally managed tablespaces. It removes the need for managing freelists and freelist groups by using bitmaps to describe the space usage of each block is within a segment. The bitmap is stored in separate blocks known as bit mapped blocks (BMBS). This relieves the contention on the segment header that occurs with freelists.
With ASSM, the linked-list freelists are replaced with bitmaps, a binary array that turns out to be very fast and efficient for managing storage extents and free blocks, thereby improving segment storage internals. Usage of ASSM causes some performance hindrance but this has been certainly overcome with the 11G Release 2.ASSM tablespaces automate freelist management by replacing the traditional one-way linked-list freelists with bitmap freelists, and remove the ability to specify PCTUSED, FREELISTS, and FREELIST GROUPS storage parameters for individual tables and indexes.
Without multiple freelists, every Oracle table and index had a single data block at the head of the table to manage free blocks for the object and provide data blocks for new rows created by any SQL insert statements. A buffer busy wait occurs when a data block is inside the data buffer cache but is unavailable because it is locked by another DML transaction. When you want to insert multiple tasks into the same table, the tasks are forced to wait while Oracle assigned free blocks, one at a time.
With ASSM, Oracle claims to improve the performance of concurrent DML operations significantly since different parts of the bitmap can be used simultaneously, eliminating serialization for free space lookups.Segment space management auto is a good method for handling objects with varying row sizes.
Large objects cannot use ASSM, and separate tablespaces must be created for tables that contain LOB datatypes. You cannot create a temporary tablespace with ASSM. This is because of the transient nature of temporary segments when sorting is performed.
Only locally managed tablespaces can use bitmap segment management.
FREELIST
Prior to Oracle 9i, each block had to be read so the freelist could be checked to see if there was room in the block. In 9i, the bitmap can be checked reducing the number of blocks read unnecessarily.
Starting from Oracle9i, the PCTUSED, FREELISTS, and FREELIST GROUPS parameters are ignored with locally managed tablespaces and ASSM.The ASSM tablespace is implemented by adding the SEGMENT SPACE MANAGEMENT AUTO clause to the tablespace definition syntax. ASSM tablespaces automate freelist management by replacing the traditional one-way linked-list freelists with bitmap freelists, and remove the ability to specify PCTUSED, FREELISTS, and FREELIST GROUPS storage parameters for individual tables and indexes.
In an Locally Managed Tablespace, Oracle moves the tablespace information out of the data dictionary tablespace and stores it directly within the tablespace itself. The second major tablespace enhancement in Oracle9i, was automatic segment space management (ASSM). a.k.a. bitmap freelists. With ASSM, the linked-list freelists are replaced with bitmaps, a binary array that turns out to be very fast and efficient for managing storage extents and free blocks, thereby improving segment storage internals.
Automatic free space management is only available in locally managed tablespaces. It removes the need for managing freelists and freelist groups by using bitmaps to describe the space usage of each block is within a segment. The bitmap is stored in separate blocks known as bit mapped blocks (BMBS). This relieves the contention on the segment header that occurs with freelists.
With ASSM, the linked-list freelists are replaced with bitmaps, a binary array that turns out to be very fast and efficient for managing storage extents and free blocks, thereby improving segment storage internals. Usage of ASSM causes some performance hindrance but this has been certainly overcome with the 11G Release 2.ASSM tablespaces automate freelist management by replacing the traditional one-way linked-list freelists with bitmap freelists, and remove the ability to specify PCTUSED, FREELISTS, and FREELIST GROUPS storage parameters for individual tables and indexes.
Without multiple freelists, every Oracle table and index had a single data block at the head of the table to manage free blocks for the object and provide data blocks for new rows created by any SQL insert statements. A buffer busy wait occurs when a data block is inside the data buffer cache but is unavailable because it is locked by another DML transaction. When you want to insert multiple tasks into the same table, the tasks are forced to wait while Oracle assigned free blocks, one at a time.
With ASSM, Oracle claims to improve the performance of concurrent DML operations significantly since different parts of the bitmap can be used simultaneously, eliminating serialization for free space lookups.Segment space management auto is a good method for handling objects with varying row sizes.
Large objects cannot use ASSM, and separate tablespaces must be created for tables that contain LOB datatypes. You cannot create a temporary tablespace with ASSM. This is because of the transient nature of temporary segments when sorting is performed.
Only locally managed tablespaces can use bitmap segment management.
FREELIST
This is a list of blocks kept in the segment header that may be used for new rows being inserted into a table. When an insert is being done, Oracle gets the next block on the freelist and uses it for the insert. When multiple inserts are requested from multiple processes, there is the potential for a high level of contention since the multiple processes will be getting the same block from the freelist, until it is full, and inserting into it. Depending on how much contention you can live with, you need to determine how many freelists you need so that the multiple processes can access their own freelist.
AUTO - This keyword tells Oracle that you want to use bitmaps to manage the free space within segments. A bitmap, in this case, is a map that describes the status of each data block within a segment with respect to the amount of space in the block available for inserting rows. As more or less space becomes available in a data block, its new state is reflected in the bitmap. Bitmaps allow Oracle to manage free space more automatically, and thus, this form of space management is called automatic segment-space management.
The package DBMS_SPACE contains a procedure called SPACE_USAGE that gives information about how space is being used within blocks under the segment high water mark.
AUTO - This keyword tells Oracle that you want to use bitmaps to manage the free space within segments. A bitmap, in this case, is a map that describes the status of each data block within a segment with respect to the amount of space in the block available for inserting rows. As more or less space becomes available in a data block, its new state is reflected in the bitmap. Bitmaps allow Oracle to manage free space more automatically, and thus, this form of space management is called automatic segment-space management.
The package DBMS_SPACE contains a procedure called SPACE_USAGE that gives information about how space is being used within blocks under the segment high water mark.
LOCALLY MANAGED TABLESPACES
Movement of the space management out of the data dictionary and into the tablespace have two benefits. First, the tablespace become independent and can be transportable (transportable tablespaces). Second, locally managed tablespaces remove the O/O contention away from the SYS tablespace.
Locally managed tablespaces with ASSM ignore any specified values for PCTUSED, NEXT, and FREELISTS specified in the table definition.
No comments:
Post a Comment