Re: A special way of migration

You stated that "The whole export will be read into undo segments". You further stated that setting consistent=y makes "the whole export 'repeatable'". These statements are mis-information.

An export, with or without consistent=y, does not consume previously unused undo space. (In fact, individual tables are always exported consistently regardless of the setting). Undo will be read as needed to create a read-consistent view of the data, just as they would for any transaction. If there is very little update activity in the database, then very little undo will need to be read.

I just started up a database and began a consistent=y export.

The export file is over 3 GB in size so far (still running).

Here is the undo usage, clearly it has nothing to do with the size of the export:

select begin_time, end_time, undoblks from v$undostat;

BEGIN_TIME END_TIME UNDOBLKS

------------------- ------------------- ----------
2005/09/28 10:14:11 2005/09/28 10:23:06          3
2005/09/28 10:04:11 2005/09/28 10:14:11         18

Gogala, Mladen wrote:

> Unfortunately, it's not a misinformation. With "consistent=y", you will
> have
> SET TRANSACTION READ ONLY as the first statement in the export. What
> does this statement do you can read for yourself in the manual. It
> wasn't an overuse of sarcasm.
>
> As far as logical backup goes, I would tend not to disagree, but it has
> nothing to do with CONSISTENT=YES. This "read consistent export" can only
> ensure consistency with respect to certain point in time if it builds a
> read-consistent image of the database with respect to that point in time.
> You have three guesses to guess which structures are used for building
> such a consistent image.
> So Mark, please read the fine manual before you accuse me of being
> overly sarcastic.
>
> --
> Mladen Gogala
> Ext. 121
>

-- 
Mark Bole
http://www.bincomputing.com



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