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Optimal freelists for high DML tables

Oracle Database Tips by Donald BurlesonNovember 17, 2015


Question:  I have buffer busy waits that are from data block contention.  Should I use ASSM and use bitmap freelists?  If not, should I add freelists or freelist groups to the table?  How many multiple freelists should I define?

Answer:  With ASSM (Automatic Segment Storage Management) a.k.a. bitmap freelists, you don't have as many buffer waits, but you may experience issues with ASSM DML throughput in super-high volume update tables.

You should define just enough extra freelists to stop the buffer busy waits, using the high-water mark of concurrent updates against the table as a guideline.  Here are my notes on Oracle freelist management.

But what if you over allocate freelists? You add additional segment header blocks to the table, and there may be a tiny increase in DML overhead to manage the freelists.  Also, Rich Niemiec has suggested that adding freelist groups instead of freelists may help.

I might suggest that you adopt an iterative process to determine the optimum number of freelists:

  1. Add freelists one at a time, during low usage times:
    alter table mytable storage (freelists 2);

  2. Measure the buffer wait changes during peak processing (statspack)

  3. Repeat until buffer waits disappear


Multiple freelist references 

According to MOSC Note: 1060377.6 we see the basic principle behind multiple freelists:

It is a good idea to always use the freelists option when creating a table that will have high inserts or updates requiring new space contention because this creates multiple process (segment) free lists instead of the default ONE master free list on a segment header.

From MOSC note: 1029850.6 we see that excessive freelists can cause sub-optimal storage utilization:

As can be seen from the algorithms above, using multiple free lists may cause some empty blocks to go unused, causing the segment to extend. If performance is critical, multiple free lists can be used to improve concurrent access, possibly at the expense of additional space used.

My multiple freelist notes:

Here are my related notes on adding multiple freelists:

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