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Oracle Locally Managed Tablespaces (LMT) benefits

Oracle Database Tips by Donald BurlesonJuly 14, 2015

Question:  I have an old database with Dictionary Managed Tablespaces and they are moving to locally managed tablespaces (LMT).  What are the benefits of LMT over DMT tablespaces?

know about the Oracle SGA and PGA, but I also hear about a RAM region called the User Global Area (UGA).  What is the UGA and how does it differ from the Program Global Area (PGA)?

Answer:  The main benefit of an LMT is that the data dictionary control structures for the tablespace extent management move from inside the data dictionary (where it's a single point of contention) into the tablespace header blocks, where there is less contention.

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.

The main benefits of locally managed tablespaces include:

  • Marginally faster performance:  Because there is less data dictionary contention, throughput can be faster.

  • Faster tablespace space management activities:  Since space management activities are done in the tablespace blocks, we see less space management contention (ST locks).  Uniform extent management is enforced and tablespace fragmentation can be reduced.

LMT's are not the default in Oracle and they also work with other tablespace features such as Automatic Segment Space Management (ASSM).  See my notes here on the benefits of LMT with ASSM

 

 
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