Re: question about automatic undo management

"Alex Filonov" <afilonov_at_yahoo.com> wrote in message news:336da121.0301220828.4b9c904d_at_posting.google.com... <snip>
> >
> > I/O activity whilst using large rollback segments
> >
> > Filename Writes
> > RBS01.DBF 686
> > USERS01.DBF 1857
> >
> > Equivalent I/O activity whilst using small rollback segments
> > Filename Writes
> > RBS01.DBF 50
> > USERS01.DBF 1813
> >
>
> I just wonder if these numbers depend on the buffer cache size? And if
> so, then how?
>

Hi Alex,

I/O activity in relation to the database buffer cache has to depend on the buffer cache size to some degree. Much has been made of the poor buffer cache hit rate recently but if the cache is too small to cope with the current demand, then we have an issue. Of course, reducing the current demand is the *key* to tuning the buffer cache and sizing rollback/undo segments appropriately is potentially of importance in reducing this demand. However, once the undo segments have been sized as efficiently as possible (eg. no significant contention, no significant growth/shrink rates, no significant numbers of unwanted 1555s) but they're still
being aged and written out to disk, then yes, we may need to look at increasing the buffer cache if practical.

Note a "way" to size undo segments with auto undo management is with the undo_retention parameter, one is not totally helpless.

Undo segments have different characteristics in relation to the buffer cache but this key point is often overlooked.

My thoughts.

Richard Received on Thu Jan 23 2003 - 05:32:57 CST