分区默认segment大小变化(64k—>8M) , 11gR2中的CELL_PARTITION_LARGE_EXTENTS 参数

CELL_PARTITION_LARGE_EXTENTS now obsolete

During the Exadata course that I am just delivering in Munich, I noticed that the fairly new parameter CELL_PARTITION_LARGE_EXTENTS is already obsolete now:

SQL> select * from v$version;BANNER--------------------------------------------------------------------------------Oracle Database 11g Enterprise Edition Release 11.2.0.2.0 - 64bit ProductionPL/SQL Release 11.2.0.2.0 - ProductionCORE    11.2.0.2.0      ProductionTNS for Linux: Version 11.2.0.2.0 - ProductionNLSRTL Version 11.2.0.2.0 - ProductionSQL> alter system set cell_partition_large_extents=true;alter system set cell_partition_large_extents=true*ERROR at line 1:ORA-25138: CELL_PARTITION_LARGE_EXTENTS initialization parameter has been madeobsolete

This parameter was introduced in 11.2.0.1 especially for Exadata Database Machine because the Allocation Unit Size (AU_SIZE) for Diskgroups built upon Exadata Cells is recommended with 4 MB. Large Segments should therefore use a multiple of 4 MB already for their initial extents. Although the parameter was made obsolete, the effect that was achievable with it is still present:

SQL> create table t (n number) partition by range (n) (partition p1 values less than (2));Table created.SQL> insert into t values (1);1 row created.SQL> select bytes/1024/1024 as mb from user_extents where segment_name='T'; MB---------- 8

I inserted before checking USER_EXTENTS because of the 11g New Feature deferred segment creation:

SQL> drop table t purge;Table dropped.SQL> create table t (n number);Table created.SQL> select bytes/1024/1024 as mb from user_extents where segment_name='T';no rows selectedSQL> insert into t values (1);1 row created.SQL> select bytes/1024/1024 as mb from user_extents where segment_name='T'; MB---------- .0625

Notice that only partitioned tables are affected by the 8 MB initial extent behavior. The new hidden parameter _PARTITION_LARGE_EXTENTS (defaults to true!) is now responsible for that:

SQL> alter session set "_partition_large_extents"=false;Session altered.SQL> drop table t purge;Table dropped.SQL> create table t (n number) partition by range (n) (partition p1 values less than (2));Table created.SQL> insert into t values (1);1 row created.SQL> select bytes/1024/1024 as mb from user_extents where segment_name='T'; MB---------- .0625

Notice that the setting of CELL_PARTITION_LARGE_EXTENTS with alter session is silently overridden by the underscore parameter:

SQL> drop table t purge;Table dropped.SQL> alter session set cell_partition_large_extents=true;Session altered.SQL> create table t (n number) partition by range (n) (partition p1 values less than (2));Table created.SQL> insert into t values (1);1 row created.SQL> select bytes/1024/1024 as mb from user_extents where segment_name='T'; MB---------- .0625

The parameter setting of the underscore parameter was still false.

SQL> drop table t purge;Table dropped.SQL> alter session set "_partition_large_extents"=true;Session altered.SQL> alter session set cell_partition_large_extents=false;Session altered.SQL> create table t (n number) partition by range (n) (partition p1 values less than (2));Table created.SQL> insert into t values (1);1 row created.SQL> select bytes/1024/1024 as mb from user_extents where segment_name='T'; MB---------- 8

Conclusion: With 11.2.0.2, partitioned tables get initial extents of 8 MB in size, which is particular useful in Exadata Environments where the ASM AU_SIZE will be 4 MB. But also ordinary Databases are affected – which is probably a good thing if we assume that partitioned tables will be large in size anyway and will therefore benefit from a large initial extent size as well.

Addendum: During my present Exadata course (03-JUN-2012), I saw a similar parameter for partitioned indexes also: _INDEX_PARTITION_LARGE_EXTENTS defaults to FALSE, though. Brief test:

SQL> select * from v$version;BANNER--------------------------------------------------------------------------------Oracle Database 11g Enterprise Edition Release 11.2.0.2.0 - 64bit ProductionPL/SQL Release 11.2.0.2.0 - ProductionCORE    11.2.0.2.0      ProductionTNS for Linux: Version 11.2.0.2.0 - ProductionNLSRTL Version 11.2.0.2.0 - ProductionSQL> create table parti (n number) partition by range (n) (partition p1 values less than (2));Table created.SQL> insert into parti values (1);1 row created.SQL> commit;Commit complete.SQL> create index parti_idx on parti(n) local;Index created.SQL> select bytes from user_extents where segment_name='PARTI_IDX';     BYTES----------     65536SQL> drop index parti_idx;Index dropped.SQL> alter session set "_index_partition_large_extents"=true;Session altered.SQL> create index parti_idx on parti(n) local;Index created.SQL> select bytes from user_extents where segment_name='PARTI_IDX';     BYTES----------   8388608

So this parameter gives us also 8M sized extents for partitioned indexes, but not by default.

原文连接：http://uhesse.com/2011/03/29/cell_partition_large_extent-now-obsolete/