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Home -> Community -> Usenet -> c.d.o.misc -> Re: Rollback Segment Setup Questions
> As a general rule, how many rollback segments should I create for my
> database and how big should I make each one? Should they all be the
> same size or does it make sense to vary the sizes? Finally, what should
> be the mix between system and public rollback segments?
Oracle has a "Rule of 4". The rule says that you take the number of concurrent transactions and divide by 4. Then round up to the nearest integer. If less than 4, round up to 4. If between 4 & 8, round up to 8. But don't use more than 50. This is the number of rollback segments you should create.
As far as sizing the rollback segments. Oracle recommends a minimum of 20 extents. You should query v$rollstat to see if your rollback segments are extending or shrinking. You want to size them in such a way that they do not experience too many shrinks or extends. This all depends on your transaction sizes. You'll want to monitor this once your system is running.
Many sites have either one or two sizes of rollback segments. Some sites have all rollback segments all the same size and configuration. It makes it easier to manage. Some sites that experience large transactions set up a couple of very large rollback segments for those big transactions, in addition to their "normal" rollback segments.
HTH,
Brian
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