by Donald K. Burleson
Over the past few releases Oracle has been automating and improving the internal administration of tables and indexes. It has gradually recognized the benefits of bitmap data structures in all areas of the database. Recently, Oracle has introduced two new tablespace parameters that automate storage management functions:
LMT (Locally Managed Tablespaces) ― The LMT tablespace is implemented by adding EXTENT MANAGEMENT LOCAL clause to the tablespace definition. LMT tablespaces automate extent management and remove the ability to specify the NEXT storage parameter. The only exception is when NEXT is used with MINEXTENTS at table creation time.
ASSM (Automatic Segment Space Management) ― The ASSM tablespace is implemented by adding the SEGMENT SPACE MANAGEMENT AUTO clause to the tablespace definition. ASSM tablespaces automate FREELIST management and remove the ability to specify PCTUSED, FREELISTS, and FREELIST GROUPS storage parameters. You cannot use ASSM unless you also use LMTs on a tablespace.
It is important to note that LMT and ASSM are optional and are used in the same instance with "traditional" tablespaces. Remember, LMT and ASSM are implemented at the tablespace level and each instance can have LMT, LMT and ASSM tablespaces, or traditional tablespaces.
Before we discuss the differences between bitmap FREELISTS and traditional FREELIST management, let's examine how bitmap FREELISTS are implemented. We begin by creating a tablespace with the segment space management auto parameter. Note that ASSM is only valid for locally-managed tablespaces with extent management local syntax.
create tablespace
asm_test
datafile
'c:\oracle\oradata\diogenes\asm_test.dbf'
size
5m
EXTENT MANAGEMENT LOCAL
SEGMENT SPACE MANAGEMENT AUTO
;
Once a table or index is allocated in this tablespace, the values for PCTUSED for individual objects will be ignored, and Oracle9i will automatically manage the FREELISTS for the tables and indexes inside the tablespace. For objects created in this tablespace, the NEXT extent clause is now obsolete because of the locally-managed tablespace (except when a table is created with MINEXTENTS and NEXT). The INITIAL parameter is still required because Oracle cannot know in advance the size of the initial table load. When using Automatic Space Management, the minimum value for INITIAL is three blocks.
There is some debate about whether a one-size-fits-all approach is best for Oracle. In large databases, individual object settings can make a huge difference in both performance and storage. As we may know, the setting for PCTUSED governs FREELIST re-linking. If we want high disk space usage, we set PCTUSED to a value xzignthly greater than avg_row_len. Conversely, if we want fast INSERT performance, we set PCTUSED to a low value, ensuring that all free blocks are nearly empty, providing lots of block space for INSERT operations.
The Issue of PCTFREE
As a quick review, the PCTFREE parameter is used to specify the amount of free space on a data block to reserve for future row expansion. If PCTFREE is set improperly, SQL update statements can cause a huge amount of row fragmentation and chaining.
The setting for PCTFREE is especially important where a row is initially stored small and expanded at a later time. In such systems, it is not uncommon to set PCTFREE equal to 95, telling Oracle to reserve 95 percent of the data block space for subsequent row expansion.
Fortunately, Oracle9i does not allow you to specify the value for PCTFREE if you are using Automatic Space Management. Row chaining is a serious problem for the DBA, and it appears that Automatic Space Management is still appropriate for tables for which you need to reserve space for large row expansions with PCTFREE.
The Issue of PCTUSED
As we know, improper settings for PCTUSED can cause huge degradation in the performance of SQL inserts. If a data block is not largely empty, excessive I/O will happen during SQL inserts because the re-used Oracle data blocks will become full quickly. Taken to the extreme, improper settings for PCTUSED can create a situation in which the free space on the data block is smaller than the average row length for the table. In these cases, Oracle will try five times to fetch a block from the FREELIST chain. After five attempts, Oracle will raise the high-water mark for the table and grab five fresh data block for the insert.
In Oracle9i with Automatic Segment Management, the PCTUSED parameter no longer governs the re-link threshold for a table data block, and we must rely on the judgment of Oracle to determine when a block is empty enough to be placed onto the FREELIST.
Unlike PCTFREE, in which Oracle cannot tell in advance how much row expansion will occur, Oracle9i does have information about the right time to re-link a data block. Because Oracle knows the average row length for the table rows (dba_tables.avg_row_Len), Oracle should be able to adjust PCTUSED to ensure that the re-linked data block will have room for new rows.
An Oracle Inconsistency
While Oracle9i ignores the PCTUSED, FREELISTS, and FREELIST GROUPS parameters with LMT and ASSM tablespaces, Oracle does not give an error message when these "ignored" parameters are used in a table definition.
SQL create table
2 test_table
3 (c1 number)
4 tablespace
5 asm_test
6 pctfree 20 pctused 30
7 storage
8 ( freelists 23 next 5m ) ;
Table created.
Most Oracle DBAs would assume that invalid parameters would be treated as they have been treated since Oracle7, and reported as an error.
SQL create index
2 test_type_idx
3 on
4 book(book_type)
5 PCTUSED 40 ;
PCTUSED 40
*
ERROR at line 5:
ORA-02158: invalid CREATE INDEX option
This could lead to confusion when a DBA believes that they are changing these values when in reality, tablespaces with LMT or SAM ignore any specified values for PCTUSED, NEXT, and FREELISTS.
No More Buffer Busy Waits
One huge benefit of Automatic Segment Management is the bitmap FREELISTS that are guaranteed to reduce buffer busy waits. Let's take a close look at this feature.
Prior to Oracle9i, buffer busy waits were a major issue. As a review, a buffer busy wait occurs when a data block is inside the data buffer cache, but it is unavailable because it is locked by another DML transaction. A block was unavailable because another SQL insert statement needed to get a block on which to place its row. Without multiple FREELISTS, every Oracle table and index had a single data block at the head of the table to manage the free block for the object. Whenever any SQL insert ran, it had to go to this block and get a data block on which to place its row.
Obviously, single FREELISTS cause a backup. When multiple tasks wanted to insert into the same table, they were forced to wait while Oracle assigned free blocks, one at a time.
Oracle's Automatic Segment Space Management feature 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.
According to Oracle benchmarks, using bitmap FREELISTS removes all segment header contention and allows for super-fast concurrent insert operations (refer to figure 1).
