Re: rollback and redo questions

Hi Doug,

I am not sure if I have understood your question correctly. There is no way you could lose information because during recovery Oracle writes all the committed transactions to datafiles from redo log files and a commit will not succeed unless it is able to write to the redo log files.

Raghu

Doug Cowles wrote:
>
> I jus ran a canned script on my rollback segments and got the following
> info
> at the bottom of the report:
>
> "Rollback contention for system undo header = 0% (Total requests = 36)
>
> Rollback contention for system undo block = 0% (Total requests = 36)
> Rollback contention for undo header = 100% (Total requests =
> 36)
> Rollback contention for undo block = 0% (Total requests = 36)
> If percentage is more than 1%, create more rollback segments "
>
> Question: What's with the undo header, and what is it? Can anyone
> explain
> why it's contention is sitting at 100%, whereas the others are at 0?
>
> Also, supplementary question - as a transaction begins, it grabs a
> rollback
> segment, right? At the same time, it places this uncommitted
> information in
> whatever is the current redo log file, right? Now, if you should lose
> the database,
> it will make use of any redo information in the redo log file, rollback
> any uncommitted
> transactions, and roll forward any committed transactions, right? So,
> if the redo
> log is smaller than the rollback segment which I'll assume is being
> maxed out,
> can't you potentially lose information IF, you have written a committed
> transaction
> to a redo log, there is a log switch for the write of the uncommitted
> transaction (using the rollback segment), and the checkpoint is not
> complete, and the database goes down, right?
>
> I know that sounds somewhat convaluted, but hopefully someone will know
> what
> I mean. Just to confirm my understanding, when a transaction commits,
> it will sit
> in the redo logs until a checkpoint occurs, at which point it is made
> permanent, right?
>
> So, I guess what I'm asking here is , under what circumstance of the
> above events
> could you actually lose something, if you get my intention.
>
> - Dc.
Received on Tue Nov 03 1998 - 21:21:37 CST