Re: why shrink rollback segs?

Thanks... good info...

On Mon, 12 Nov 2001 06:45:21 -0800, you wrote:

>Generally I try monitor v$filestat to see what
>proportion of IO is due to rollback...The ideal is of
>course nil, but to do so pretty much means giving
>everyone a couple of dozen 1555's every day :-)
>
>So you end up with the balancing act of trying to make
>them as small as possible without introducing 1555's
>
>hth
>connor
>
> --- Doug C <dcowles_at_i84.net> wrote: > Thanks.. makes
>sense...
>> Can I use x$bh or something to determine whether
>> it's nicely squeezed into the
>> buffer at some point in time?
>>
>>
>> On Mon, 12 Nov 2001 02:00:21 -0800, you wrote:
>>
>> >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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>> >> --
>> >> Author: Doug C
>> >> INET: dcowles_at_i84.net
>> >>
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>> Author: Doug C
>> INET: dcowles_at_i84.net
>>
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>=====
>Connor McDonald
>http://www.oracledba.co.uk (mirrored at
>http://www.oradba.freeserve.co.uk)
>
>"Some days you're the pigeon, some days you're the statue"
>
>__________________________________________________
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--
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Author: Doug C
  INET: dcowles_at_i84.net

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