Re: BaanIV and rollback segments

I think that Baan databases have a wide range of transaction profiles, so optimal rollback segment layouts from other Baan sites may not be any more effective that your own current one.

The Oracle engine tries to avoid overwriting blocks in the rollback segments as long as possible. So, if you are having "snapshot too old" errors, it is not consistent to say "there is no contention" and "there is enough space."

In my understanding, this is what happens in these errors. Any time an implicit or explicit query starts, the engine associates that query with the current SCN (system change number). Each time the query needs to read a data block, the SCN of that block is checked. If the block's SCN is higher (newer) than the query's SCN, it means the block has been modified after the query started. The engine tries to find that block in the rollback segments with an appropriate SCN (this is how the guarantee of read consistency is maintained). If that block, with an appropriate SCN, is no longer in the rollback segments (i.e., it has been overwritten by data blocks from newer transactions), the error occurs.

OK, you have 10 rollback segments of 60Mbytes each. Still, queries on your Baan database are sometimes taking so long that data blocks they need are already overwritten by more recent modified blocks in the rollback segments. It is understood that you can't do anything about Baan's query nor point transactions to any particular rollback segment.

I would still give the rollback segment layout I recommended a try. Even if you don't want to allocate more than 600Mbytes to rbs space, I would change to 8 rollback segments with an optimal (and minextents) size of 60Mbytes each. It is important to have each segment's starting size equal the optimal size. There is no sense in making the segment add extents on the fly to reach its optimal size. Set maxextents to allow using the 120Mbytes of free space -- for the segments to extend into, before reusing the space occupied by data blocks that still may be needed by long-running queries.

Good luck.
Roger

Han Brinkman wrote in message <898071770.306659_at_jive.news.big-orange.net>...
>What I have seen untill now that there is no contention around the rollback
>segments. Also the segments themselves don't grow dynamically, so there is
>enough space. It's just a matter of that old data is overwritten which
>another query still wants to have.
>
>What I would like to know is how other Baan sites have configured their
>rollback segments.
>
>Han
>
>
Received on Fri Jun 19 1998 - 11:57:48 CDT