Re: why shrink rollback segs?

From: Connor McDonald <hamcdc_at_yahoo.co.uk>
Date: Mon, 12 Nov 2001 02:29:01 -0800
Message-ID: <F001.003C2131.20011112020020@fatcity.com>

If rb's are small, then there is a good chance that just when they are about to aged out of the buffer cache, hey presto! another session needs them again - so you end up with a nice scenario where they spend their life in the cache and cause virtually no IO.

As they get larger and larger, the time-before-reuse time gets longer and longer - which will mean more and more IO. Its the same for having lots of extents - many moons ago this used to be due to txns needing their own extents. Oracle lifted this restriction somewhere in v7, but it still makes sense to have 20 or so extents, because then when a rollback segment increases in size, it does it in nice 5% increments - keeping the probability of it all fitting in the cache high.

hth
connor

• Doug C <dcowles_at_i84.net> wrote: > OK.. this may be a little naive..and maybe I'm just
  > jogging my own memory, but
  > here it goes...
  > There are a couple schools of thought on rollback
  > segments as near as I can
  > figure. Some like to set optimal, and some don't.
  > The one's that don't like
  > to shrink them at night or once daily or some other
  > variation..
  >
  > Question: Why bother? If they get big and you have
  > the room, so what? They
  > won't have to extend tomorrow, no? If you're
  > concerned about space, why not set
  > a good maxextents or a limit on the filesize..
  >
  > What's the problem with them getting large?
  >
  > Thanks,
  > Doug
  >
  >
  >
  >
  > --
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Connor McDonald
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Received on Mon Nov 12 2001 - 04:29:01 CST