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RE: Redo log question

From: Allen, Brandon <>
Date: Thu, 22 Sep 2005 15:43:06 -0700
Message-ID: <04DDF147ED3A0D42B48A48A18D574C450236163F@NT15.oneneck.corp>

See comments below, copied from 9i concepts guide (

> -----Original Message-----
> From:
> []On Behalf Of Peter Dixon

. . .

> If the instance crashed then,
> on recovery
> SMON would firstly reconstruct the undo from the change
> vectors to the
> rollback block and reconstruct the changes to the data blocks
> but would then
> rollback the changes because the transaction table slot in
> the rollback
> segment header shows that this transaction is not comitted.
> Is this correct?

Yes, I believe you are correct.

> OK moving on - what exactly happens when the user issues a
. . .

> So when rolling forward using the online redo log files, for this
> transaction we have:
> 1) rollback segment header(uncommitted)
> 2) change vectors to reconstruct "before image" undo blocks
> 3) change vectors to change data blocks
> 3) rollback segment header(committed)
> I'm sure this is an over simplified (and probably wrong) version. Can
> someone please clarify this for me?

Not sure about that, I can't find any detail on exactly what the contents of the "commit record" are, but here is what is says in the doc:

"When a user issues a COMMIT statement, LGWR puts a commit record in the redo log buffer and writes it to disk immediately, along with the transaction's redo entries."

> My other major area of confusion in all of this is where do
> SCN's fit in,
> when are they allocated?

"When a user commits a transaction, the transaction is assigned a system change number (SCN), which Oracle records along with the transaction's redo entries in the redo log. SCNs are recorded in the redo log so that recovery operations can be synchronized in Oracle9i Real Application Clusters and distributed databases."


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Received on Thu Sep 22 2005 - 17:43:37 CDT

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