Oracle ASSM Performance Oracle Tips by Burleson Consulting

Performance Issues With ASSM

The Oracle community has mixed feelings about using ASSM tablespaces.

Among the top points about ASSM, there are both pros and cons:

Pros of ASSM:

§         Varying row sizes: ASSM is better than a static pctused. The bitmaps make ASSM tablespaces better at handling rows with wide variations in row length.

§         Reducing buffer busy waits: ASSM will remove buffer busy waits better than using multiple freelists. When a table has multiple freelists, all purges must be parallelized to reload the freelists evenly, and ASSM has no such limitation.

§         Great for Real Application Clusters: The bitmap freelists remove the need to define multiple freelists groups for RAC and provide overall improved freelist management over traditional freelists.

Cons of ASSM:

§         Slow for full-table scans: Several studies have shown that large-table full-table scans (FTS) will run longer with ASSM than standard bitmaps. ASSM FTS tablespaces are consistently slower than freelist FTS operations. This implies that ASSM may not be appropriate for decision support systems and warehouse applications unless partitioning is used with Oracle Parallel Query.

§        Slower for high-volume concurrent inserts: Numerous experts have conducted studies that show that tables with high volume bulk loads perform faster with traditional multiple freelists.

§         ASSM will influence index clustering: For row ordered tables, ASSM can adversely affect the clustering_factor for indexes. Bitmap freelists are less likely to place adjacent tows on physically adjacent data blocks, and this can lower the clustering_factor and the cost-based optimizer's propensity to favor an index range scan.

Also see MOSC Note: 6781367.8 and Bug 6781367 ALTER TABLE ADD COLUMN or mass UPDATE can be slow in ASSM.
 

The combination of bitmap freelists, ASSM, and Locally Managed Tablespaces has greatly simplified and improved the internal management of data blocks within Oracle tablespaces. The use of bitmap freelists removes the need to define multiple freelists for tables and indexes that experience high volume concurrent DML and provides a simple solution to the problem of segment header contention.

However, the savvy DBA recognizes the tradeoff between one-size-fits-all convenience and the power of being able to set individual object parameters for tables and indexes. The choice of LMT and ASSM for tablespace management depends heavily on the application, and real-world Oracle tablespaces will implement LMT and ASSM tablespaces only after careful consideration.

The following section examines proactive tablespace management techniques on Oracle10g.

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