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RE: Rollback Segments

From: A. Bardeen <abardeen1_at_yahoo.com>
Date: Wed, 09 May 2001 10:23:24 -0700
Message-ID: <F001.002FE110.20010509094207@fatcity.com>

Kevin,

This is a common misconception.

On startup Oracle determines the number of rollback segments to bring on line based on the init.ora parameters:
  TRANSACTIONS / TRANSACTIONS_PER_ROLLBACK_SEGMENT If the number of rollback segments specified by ROLLBACK_SEGMENTS is less than that value, then Oracle automatically will bring online any other PUBLIC rollback segments to bring the total of online rollback segments up to that value. Any PUBLIC rollback segments remaining will be left offline. PRIVATE rollback segments will only be brought online automatically if specified in the ROLLBACK_SEGMENTS parameter.

Misconception #2: TRANSACTIONS_PER_ROLLBACK_SEGMENT determines the number of concurrent transactions that can use a rollback segment. Granted that's what the name implies, but this isn't the case. This parameter is used only to determine the number of PUBLIC rollback segments to bring on line automatically at startup. The number of concurrent transactions that can use a rollback segment is based on many factors, the most important one being DB_BLOCK_SIZE.

I recommend using private rollback segments and specifying them in ROLLBACK_SEGMENTS because it makes it much easier to handle rollback segment problems. If there's a problem with a rollback segment on startup, for example, simply removing the segment from the init.ora will usually allow you to at least get the db open.

HTH,


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Author: A. Bardeen
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Received on Wed May 09 2001 - 12:23:24 CDT

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