How recovery works after an online backup

Hello

Although conceptually tent understand how backup and restoration works, I stumbled upon a question about the hot (on-line) backup.
It is a conceptual question (I'm trying to understand how things work), is not a ' how can I do / what should I do step by step "question.

As far as I understand, an online backup of a tablespace can be done by copying files operating system place a tablespace while the database is in place and used (i.e. transactions are changing data in the database). Before the copy of the operating system starts, the Oracle RDBMS should be informed that an online backup is taken through 'ALTER '.... BEGIN "(such as additional information are written to the log again, that may be required for subsequent recovery using the backup online BACKUP). During recovery, the Oracle RDBMS uses copies of the OS as well as the line and archived files redo logs in order to rebuild all committed transactions and it uses more (uncommitted) transactions ongoing restoration UNDO tablespace.

Think about it, it seems to me that, so that it works in all possible scenarios to cancel it, information from moment has been the backup may be required. So the UNDO tablespace backup suits as well (see the explanation of this assumption below). However browsing the internet (including the Oracle online documentation) I find all instructions for the backup of the UNDO tablespace when an online backup is taken. More I couldn't understand when exactly such a backup of the UNDO tablespace must be done to ensure that the database can be retrieved in all scenarios.

I believe that the cancellation as soon as the last hot backup information can be held for example in the following scenario:
Suppose we take a backup of a given tablespace hot, i.e. we copy all files of the OS that make up this tablespace, while the database is potentially used. Either D1 one of the data files in the space of our table and let the T1 transaction change datafile D1. Allow transaction T1 still be engaged during the copy of datafile D1 are made and allow of (at least part of) T1 changes included in backup D1' D1 (because DBWR has already written the changed blocks to the moment where they have been copied to the backup). Allow transaction T1 be restored once the copy is complete. D1' will therefore include changes in T1, while D1 do not.

Now some time later, the D1 data file is lost. When you get the copy of D1 D1 ", recovery logs (archived) apply to D1. Before that, transaction T1 must be restored in the copy of D1 ", because changes to the T1 should not appear in the version retrieved from the database.
I have however not, understand the information in the statement rollback transaction T1 out exactly. It can still be in the current UNDO tablespace. I guess however that rollback information is not maintained in the UNDO tablespace forever. I see three possible answers to this
(a) there are some requirements that missed me so far for the backup UNDO tablespace whenever a hot backup is found.
(b) given that the Oracle "RDBMS" must be notified that you made an online backup, it can store all the information relevant cancellation (e.g. write it in the redo log) when the tablespace is put in backup mode.
(c) there are situations where recovery is not possible due to the "old UNDO information missing.

Answer (b) seems the most plausible to me. However, I found no confirmation of this and if (b) is really the answer, I'd be interested to understand what information is stored where by the RDMBS Oracle and how it is used for recovery.

To summarize, I have the following questions:
(I) is there any obligation to save the UNDO tablespace and an online of a tablespace backup, and if so, where is this stated in the Oracle documentation?
(II) mechanisms to ensure that uncommitted transactions can be identified the copy a tablespace online (potentially very long once the copy has been made)?
(III) you know all the links (Oracle documentation or other online resources) explaining these datails?

Thank you for any advice and answers
Kind regards
Martin

However I don't see how T cancellation of transaction information can be recovered without such a backup of the UNDO tablespace

Taking your scenario that only datafile D is lost and only this data file must be restored when the loss is discovered in time t8:

At the time the restored datafile D t9 is from t4.

However, Oracle notice now that, while the rest of the database (i.e. all the other data files) from t9, this data file is from the t4. This cannot be permitted (a data file cannot be inconsistent with the rest of the database unless it is offline). That's why Oracle requires you to RECOVER DATAFILE D.
Get it BACK then if applies to all again them (archivelogs) T4 starting until t9 that relates to this data file.
The price drop happened in the t6 is also re-played - of the redo (no cancellation). {Don't forget that again picking up all of the changes, including restorations! so the cancellation at the t6 was also captured in it again!}
For example, the data file is presented in a consistent state without having to redo cancel them.
No matter if the information contained in the cancellation had been released and crashed between t6, t8 and t9.

When Oracle made a recovery of media to a data file, there no need to cancel - media recovery does not care if the blocks are table, index, LOB, transactions, commits, restorations; It was simply a record physical of all the changes.

PS: This has nothing to do with what version you use V6 11.2
If the tablespace is called 'UNDOTBS1' or 'RBS' is irrelevant.

Hemant K Collette
http://hemantoracledba.blogspot.com

Published by: Hemant K grapple Sep 28, 2010 22:16

Published by: Hemant K grapple Sep 28, 2010 22:21

Published by: Hemant K grapple Sep 28, 2010 22:22

Tags: Database

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