How does recovery work after an online backup

Question

Hello,
While trying to conceptually understand how backup and recovery works, I came accross a question concerning hot (online) backup.
This is a conceptual question (I am trying to understand how things work), it is not a "how should I proceed/ what should I do step by step" question.
As far as I understand, an online backup of a tablespace can be performed by copying the OS files making up a tablespace while the database is up and being used (i.e. transactions are modifying data in the database). Before the copying of the OS files starts, the Oracle RDMS must be notified that an online backup is being taken via "ALTER...BEGIN BACKUP" (such that some additional information is written to the Redo Log, which may be required for subsequent recovery using the online backup). During recovery the Oracle RDBMS uses the copies of the OS files together with the online and archived redo logs in order to reconstruct all committed transactions and it further uses the UNDO tablespace to rollback open (uncommitted) transactions.
Thinking about this, it seems to me, that in order for this to work in all possible scenarios the undo information from the time the backup was taken may be required. Therefore backup of the UNDO tablespace should be taken as well (see the explanation for this assumption below). However browsing the internet (including the Oracle online documentation) I did not find any statements concerning the backup of the UNDO tablespace when an online backup is taken. Moreover I couldn't figure out when exactly such a backup of the UNDO tablespace must be done, to ensure that the database can be recovered in all scenarios.
I believe that undo information from the time the hot backup was taken may be required e.g. in the following scenario:
Assume we are taking a hot backup of a given tablespace, i.e. we are copying all OS files that make up this tablespace, while the database is potentially being used. Let D1 be one of the datafiles in our tablespace and let transaction T1 modify datafile D1. Let transaction T1 further be uncommitted while the copy of datafile D1 is being made and let (at least some of) the changes from T1 be included in the backup copy D1' of D1 (because DBWR has already written the modified blocks at the time they were being copied to the backup). Let transaction T1 be rolled back after the copy is completed. D1' will thus contain modifications from T1, while D1 will not.
Now some time later the datafile D1 is lost. When recovering D1 from the copy D1', the (archived) redo logs will be applied to D1'. Before that, transaction T1 should be rolled back in the copy D1', because modifications from T1 must not appear in the recovered version of the database.
I do however not understand, where the information to rollback transaction T1 exactly comes from. It may still be in the current UNDO tablespace. I do however assume that rollback information is not kept in the UNDO tablespace forever. I see three possible answers to this
(a) There are some requirements which I missed so far to backup the UNDO tablespace whenever a hot backup is made.
(b) Since the Oracle "RDBMS" has to be notified that an online backup is being done, it might store all relevant undo information (e.g. write it to the redo log) when the tablespace is put in backup mode.
(c) There are situations when recovery is not possible due to "missing old UNDO information".
Answer (b) seems the most plausible to me. I did however not find any confirmation of this and if (b) really is the answer, I would be interested to understand what information is stored where by the Oracle RDMBS and how it is used for recovery.
To summarize I have the following questions:
(I) Is there any requirement to backup the UNDO tablespace together with an online backup of a tablespace, and if so, where is this stated in the Oracle documentation?
(II) What mechanisms ensure that uncommitted transactions can be cleared from the online copy of a tablespace (potentially a long time after the copy was taken)?
(III) Do you know any links (Oracle documentation or other online resources) explaining these datails?
Thank you for any hints and answers
kind regards
Martin

Answer 1

Its a highly technical question and I can be completely wrong due to my very less knowledge but I would attempt to answer still. Hope I say something sensible so bear with me.
As far as I understand, an online backup of a tablespace can be performed by copying the OS files making up a tablespace while the database is up and being used (i.e. transactions are modifying data in the database).Correct. But it would depend on the tool you are going to use to do so.Using o/s level commands like CP and all would require you to manually copy the files to the backup location. Using RMAN, it would be lot easier and RMAN would take care of everything.
Before the copying of the OS files starts, the Oracle RDMS must be notified that an online backup is being taken via "ALTER...BEGIN BACKUP" (such that some additional information is written to the Redo Log, which may be required for subsequent recovery using the online backup). Again, this is a requirement only in the case of user-managed backup . In that case, because of the fractured block issue , its important that the corresponding older information/image of the buffer is also copied in the redo stream and that's done when the begin backup command is used. Using RMAN, this is not needed as RMAN can read the consistent image which it would store in the backup piece, exactly in the same way in which select request is fulfilled by oracle for a dirty buffer which is yet to be made consistent.
During recovery the Oracle RDBMS uses the copies of the OS files together with the online and archived redo logs in order to reconstruct all committed transactions and it further uses the UNDO tablespace to rollback open (uncommitted) transactions.Correct!
Thinking about this, it seems to me, that in order for this to work in all possible scenarios the undo information from the time the backup was >taken may be required. Therefore backup of the UNDO tablespace should be taken as well (see the explanation for this assumption below). >However browsing the internet (including the Oracle online documentation) I did not find any statements concerning the backup of the UNDO >tablespace when an online backup is taken. Moreover I couldn't figure out when exactly such a backup of the UNDO tablespace must be done, to >ensure that the database can be recovered in all scenarios.The reason that its not a must to do so is that if the transaction is yet active, there is no way that Oracle would overwrite the Undo information of it, even if you may come after 100 years, it would be there. The Undo segment would mark those blocks as active undo blockswhich contains the information of that transaction whose status within the transacton table of that undo segment is still marked as active. So its there all the time in the undo tablespace. Now, for an instance, let's assume that the undo is not there as well( it would be but let's assume), the changes made to the undo segment's blocks are also recored in the redo as its just a change happening to any other segment like EMP,DEPT except with the difference that its not done by you but by oracle. So using that information, in the future , if there would be a need to replay those changes, necessary information to do so can be brought up from the redo blocks stored in the redo/archive logs. Yes, if there would be pending transactions that would require Undo information to get them rolled back and you have lost Undo tablespace and have no backup of it , you wont be able to bring back the database as it would be inconsistent and oracle would not let you to do it. In that case, you may require to use hacks to get it up and that would be really tricky situation.
(I) Is there any requirement to backup the UNDO tablespace together with an online backup of a tablespace, and if so, where is this stated in the Oracle documentation?As I said above, it must be there if you are anticipating loss of Undo tablespace. If you have lost it, you would need a backup and all the archive logs and redo logs to recover and get it back to the point where the current database is . Rest, oracle would take care as it would reapply the redo contents of the undo segments over the undo segment and get it consitent.
(II) What mechanisms ensure that uncommitted transactions can be cleared from the online copy of a tablespace (potentially a long time after the copy was taken)?As I said , pending transaction's undo is never overwritten by Oracle. Its always kept and marked as active undo . Only a transaction end would make it elgible to get overwritten and that too won't happen immediately(undo_retention would kick in) .
(III) Do you know any links (Oracle documentation or other online resources) explaining these datails?I have to see if its there some where step by step mentioned and I shall update the reply once I shall find the link. Hoep someone else in the meantime finds it .
HTH
Aman....