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Re: What exactly is writen in the logs.

From: Jan Syssauw <jsyssauw_at_village.uunet.be>
Date: Thu, 18 Mar 1999 21:09:53 +0100
Message-ID: <7crmdo$hvj$1@xenon.inbe.net>


No quite right

redo log are written by the LGWR and as the word says these files (together with the archives-files) contain all the information to redo every action ever done in the database in case of (media) recovery or instance recovery
This contains the information needed to do the roll-forward Rollback segments contains the before imagine of something that is changing for 2 purposes
1) if the transaction fails to do the rollback of the information, getting it

     back into the state as it was before 2) read consistent view

The growing of a rollback is in fact not really related to this, normally a well dimensioned rollback segment should 'never grow or shrink' every 'normal' transaction should have space enough inside a rbs the rbs-segments are used in a circular way, for example take a rbs of 3 extents (a, b, c) a transaction start using our rbs, for example at extents a, when a is filled, it will check if there is space left in the next one ,meaning is there no other transaction going on overthere that used up all the space in extent b, then it will jump to extent b and use it space to continue, and so on
If no space left in the next extent, because an active transaction (not commited yet) is
taking all space, then will the rbs need to allocate a new extent (=growing), but will
try to shrink at the same time the oldest extent not in use by an active transaction,
and only if this isn't possible because all extents are in use, the rbs segment will
grow.

Regards
Jan

PS if you're interested to see what's inside the redolog and archives there a new utility that comes with 8.1 that's called the logminer This can be used to analyze the redologs of 8.0 and 8.1 versions.

Any comment is welcome

Doug Cowles wrote in message <36EE7474.E8EEC667_at_bigfoot.com>...
>If I may once again try to interpret things in this interesting thread.
>First of all, to clarify a before and after image of a rollback. A
rollback
>either grows, logging before images, or shrinks, rolling back by rewriting
>before images of data blocks. I take it, a before image of the rollback
>is when it grows, and an after images is when it rolls back.
>Is this right?
>
>If this is right, then only a rolled back transaction (after images of
rollback
>segments), actually makes it into the redo logs. Is this right?
>
>So, if you lose your rollback segment (which contains uncomitted
transactions),
>you cannot rebuild either the rollback segment, or recover properly,
because
>
>1) The redo logs have logged information as if the transaction was
committed.
>2) And, you have no record of the before image because you have
> a)lost the roll back segment
> b)since the rollback was never "rolled back", you don't have the after
(before)
> images of your data in the redo logs.
>
>Is this why a message to call technical support usually shows up if you
lose
>a rollback segment?
>
>What do they do for you anyhow?
>
>I concur by the way, this has been an epiphany .. thanks so much for the
info.
>- Dc.
>
>Thomas Kyte wrote:
>
Received on Thu Mar 18 1999 - 14:09:53 CST

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