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 lets 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 locallymanaged tablespaces with extent management local syntax
create tablespace
asm_test
datafile
c:\oracle\oradata\diogenes\asm_testdbf
size
m
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 Oraclei 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 locallymanaged 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 onesizefitsall 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 relinking 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 telling Oracle to reserve percent of the data block space for subsequent row expansion
Fortunately Oraclei 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 reused 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 highwater mark for the table and grab five fresh data block for the insert
In Oraclei with Automatic Segment Management the PCTUSED parameter no longer governs the relink 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 Oraclei does have information about the right time to relink a data block Because Oracle knows the average row length for the table rows (dba_tablesavg_row_Len) Oracle should be able to adjust PCTUSED to ensure that the relinked data block will have room for new rows
An Oracle Inconsistency
While Oraclei 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
test_table
(c number)
tablespace
asm_test
pctfree pctused
storage
( freelists next m ) ;
Table created
Most Oracle DBAs would assume that invalid parameters would be treated as they have been treated since Oracle and reported as an error
SQL> create index
test_type_idx
on
book(book_type)
PCTUSED ;
PCTUSED
*
ERROR at line :
ORA: 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 Lets take a close look at this feature
Prior to Oraclei 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
Oracles 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 superfast concurrent insert operations (refer to figure )
