Undo behavior during instance recovery

i'm confused about 2 concepts in applying undoin the case of instance failure
1.the common concepts of instance recovery which states :
a) when instance crash, oracle automatically applies all redo in the redo file to roll the db forward to the last scn be4 the failure
b) & redo data also contains the undo data, it also undo the uncommitted changes from the old valuesin the undo.
that'sok
2.but refereing to 10g database concepts (ch17 high availability-p317 if pdf)
url: http://download-uk.oracle.com/docs/cd/B19306_01/server.102/b14220/high_av.htm#sthref2531
it says:
With fast-start fault recovery, the Oracle database is opened for access by applications without having to wait for the undo, or rollback, phase to be completed.
The rollback of data locked by uncommitted transaction is done dynamically on an as needed basis.
If the user process encounters a row locked by a crashed transaction, then it just rolls back that row.
The impact of rolling back the rows requested by a query is negligible.
Fast-start fault recovery is very fast, because undo data is stored in the database, not in the log files.
Undoing a block does not require an expensive sequential scan of a log file.
so which situation is applicable in case of instance recovery
& do the undo behvior stated above in case of fast start fault recovery is an option or that's the default
thnx alot

Hi,
Whatever changes are made to the database, are recorded in redo log buffer cache sequentially and then shifted to redo log files. Suppose redo logs recorded following changes in the database in following sequence.
insert a row
update a row
commit;
delete a row
Now Instance crashes before these changes were written into the database (data remains in buffer cache until checkpoint)
Now during instance recovery, data which was still in buffer cache and not written to the data files, will be applied in same sequence. All undo data for update/delete is in redo log files and will be applied in the same sequence.
Salman

Similar Messages

Undo tablespace and instance recovery

Is UNDO tablespace is mandatory during instance recovery. ?
Suppose my undo tablespace has some active transaction and instance crashes making undo tablespace inaccessible.
Will database search for that undo tablespace during startup or I can simply create a new one and set as new undo tablespace ?
With Regards
Amt

Hi,
>>I am actually confused
Keep in mind that If an Oracle instance crashes, any changes that are made in the SGA are not written to the data files. When you restart the instance, the SMON background process automatically performs instance recovery by performing the following tasks:
• Rolling forward changes that are made in the online redo log files but not in the data files. Since all the committed transactions are written to the online redo log files, these are successfully recovered as result of rolling forward changes from the online redo log files to the data files.
• Opening the database. After the database is opened, users can log on and access any data that is not locked by un-recovered transaction.
• Rolling back all the uncommitted transactions.
In resume, during database recovery, undo records are used to undo any uncommitted changes applied from the redo log to the datafiles.
But if your UNDO tablespace crashed, then you will need recover it, using steps like
1) shutdown abort (If the instance yet is up)
2) cp -a /backup/undo01.dbf /u01/oradata (restore the latest recent backup file)
3) startup mount
4) recover database or recover datafile '/u01/oradata/undo01.dbf'
5) alter database open;
In some cases, re-create it, also resolve the problem:
http://asktom.oracle.com/pls/asktom/f?p=100:11:0::::P11_QUESTION_ID:5669213349582
Missing UNDO tablespace in restore backup
>>I checked in test setup and it is actually waiting for missing UNDO tablespace.
Which exactly test you are doing?
Cheers

Instance recovery

SMON process perform instance recovery from online redo log files.
does this means that data in undo segment, is not available at next instance startup, is it only available in memory?
if undo data is available in undo tablespace, why redo log files are used to perform instance recovery?

Tell me did you try find the answer or explanation in documentation first? I don't think so.
Oracle is one of best documented database (IMHO) and there are still people who are omitting this good documentation (honestly I don't understand why they do that).
Now back to your question.
During instance recovery there are two phases:
rollforward phase (cache recovery) - applying informations from redo logs
rollback phase (transaction recovery) - rollback all uncommited transactions. This phase uses undo segments (if undo management = auto) or roll back segments (if undo management = manual).
Maybe you should try read more documentation rather than books with lack explanation.

Oracle Concepts: Clarity required on Instance Recovery

One of my colleague was discussing instance recovery with me.
We discussed about checkpoint position, instance recovery phases (rolling forward & rolling back ), then he asked me whats the point of rolling forward (applying redo logs) and then rolling back (applying undo blocks), why not applying only committed transactions?
I told him that this happens because:
1) roll forward to recreate the scenario just before when Instance crashed
2) roll forward/roll back is the fastest way to recover
I am not sure from where I got this learning, may be some blog or article (probably not from Oracle Docs), but is it correct?
Regards,
Ankit Rathi

OnB wrote:
The 2nd point is completely wrong!Let me elaborate the context of 2nd point, it was mentioned in context of applying only committed transactions during recovery. As all the change vectors go in the redo log files, had there been any mechanism to identify only committed transactions, it would have taken more time (its really hypothetical as there is no mechanism exists, right?) by that mechanism to identify committted transactions (from redo logs) than simply rolling forward and rolling back.There is no context in that point, it's wrong , plain and simple!
There is a mechanism to identify committed transactions and that's called commit . But when you are doing recovery, you are not recovering just the committed transactions but you are recreating the entire scene before the crash which means, you would be required to apply both the uncommitted and committed work/change vectors. Going by your logic, if Oracle would start searching for the committed transactions, it would have many issues. One, time taken would be more higher. Two, there may be an instance that the commit was issued but before the commit complete message could reach the user, instance got terminated so how about those blocks which may be now in data file and have got an active transaction status and are unsure whether they are committed or not. And last, the way LGWR writes is just to keep on pushing change vectors to the redo stream in the order of the SCN. So for doing a recovery, you would need to reapply all of that changed data in that same SCN ordering.
The meaning of instance recovery is that you are not having a physical file crashed or deleted but there was a memory loss and few buffers were left out from being written to the data files and now, data files , control file and log files are not in synch.
The second point, where you are saying its the fastest, is wrong because even for the instance recovery , it can take a very long time and it would depend on how much work was left over before the instance was crashed. Roll Forward is the way to bring the data files in synch with the redo log files and the control files. So if the data files were on Checkpoint Number 100 and the control files and redo log files recorded the number as 120, the missing 20 numbers gap would be reentered in the data files because only then, the database would be opened. Now, your friend's question that why not only committed transaction because you never say a transaction is only done when there is a commit. Whenever any DML is encountered , it's going to cause a transaction. And you do want that missing work to be recreated to ensure that if there was a commit issued before the instance was crashed , next time, Oracle would ensure that work is there and if not , it would be rolled back. And most importantly, in the redo log files all the change vectors go, not just for the committed transactions .This is fine, same understanding here.
Bring in your own thoughts now that how much you understood from this explanation before I start to get into more technical terms.I would like to know your view about the 1st point. Suppose, something went wrong (in terms of transactions) due to which instance crashed, if it rolls forward (as it happens automatically) during next start-up, will it crash again?
It would be rolled back in the case of the instance (next) startup. If all what you are doing is plain DML's they won't crash the instance as long as you haven't encountered any bug in the case of which, next time also the instance may crash.
Aman....

Instance recovery by SMON

Hi,
are you agree with the followings :
Your database is running in the ARCHIVELOG mode. After an instance crash, you start up the database by issuing the STARTUP
command. Instance recovery is initiated by the SMON background process.
The information used for instance recovery will be derived from the control file.
The uncommitted changes are rolled back using information in the undo segments.
The committed and uncommitted changes stored in the online redo log are applied to the
affected data blocks.Many thanks.

Oracle instance recovery. I am remebering, one day i was talking with one of my the best and great oracle expert friend and i asked him; "Suppose you have got a chance to ask one question from OCM; what will you ask? He said; i will ask him explain how and when oracle perfrom instance recovery". So you can judge what is instance recovery and how much lengthy is this topic. But if i have to explain then first i will collect some notes from forum and google and the text will be as follow :
1. The recovery is triggered by smon comparing thre control files and data file headers. If the stop scn of the datafile doesn't match with the checkpoint scn of control file, recovery is triggered(hope I remember correctly). After that recovery starts. There are changes in it from 9i but don't think its neded here. Read on the docs!
Source:Re: instance recovery in oracle 9i
Writer:Aman....
Please read replies by Kamran and Pavan in the above thread.
2. There is no Instance Recovery at the STARTUP and MOUNT phases. Instance Recovery is at the OPEN phase.
Source:how oracle will come to know weather it is Instance recovery or Media Recovery
Writer:Hemant K Chitale
3. Re: crash recovery/ instance recovery
Writer:Eduardo Legatti
etc. etc.
I mean if i wish to write a note on instance recovery i will collect some good and clear replies by forum experts (as i showed example above) and then will shape-up in my language and then; i will post that text in the forum; so that necessary correction too can be performed (if required).
Regards
Girish Sharma

Indesign CS6 quits during document recovery, even after restart. How do I get out of this cycle?

Indesign CS6 quits during document recovery, even after restart. How do I get out of this cycle?
I was saving a document when it originally quit. so "locked" file appeared in directory as well as reg file.
Upon restart of InDesign, recovery failed and software quit again.
Restarted computer, then indesign, file opens but as soon as I select "recover" it sends a message that indesign needs to close.
I cannot seem to get out of this cycle. Help.

Use your operating system’s Search utility to find a folder called (InDesign Recovery) and delete it.

Instance recovery mechanism

Hi,
I try to understand the mechanism of instance recovery of Oracle 8i/ Oracle 9i database and I am little bit confused, since I got some opposed information about that.
Please find below my questions:
1. Is SCN number assigned to all transactions or ONLY to committed transactions?
2. Is the checkpoint number the highest SCN number after an checkpoint event?
3. After checkpoint event, dirty buffer are written to database files by DBWR. Does DBWR write only dirty buffers of committet transactions or does DBWR write out all dirty buffers in the DB buffer cache?
Thanks in advance for your answers!
regards
Peter

1. The system change number (SCN) is updated whenever there is a commit.
2. If you are talking about the checkpoint_change# in the v$database table, yes
3. All dirty buffers are written to disk. Data files can and do contain uncommitted data
Justin
Distributed Database Consulting, Inc.
http://www.ddbcinc.com/askDDBC

Purpose of ONLINE REDO LOG FILES - Media or Instance recovery or BOTH ?

Hi
Currently studying this topic for the 1z0-031 exam and am a little confused.
my books (from instructor led class) say
-redo logs are a mean to provide redo transactions in the event of a DATABASE recovery
-redo log buffer gets flushed to redo log files to provide a recovery mechanism in case of MEDIA FAILURE
Then it says
-Online redo log files are used in a situation such as an INSTANCE FAILURE to recover uncommitted data which has not yet been written to the data files
- online redo log files are used for RECOVERY only.
Am i misunderstanding? Or are redo log files for both MEDIA and INSTANCE recovery? Or just INSTANCE ?
confused....
Amanjit

Online Redo Log Files are used in a sense for both Media and Instance Recovery. If your database is in NoArchive Mode then you will only be able to use the Redo Log Files for instance recover. But if you are running in Archive Log Mode then Redo Log Files are archived and will allow you to recover from media failure.

Uncommited transactions remain after Instance Recovery

After Instance Recovery, the database seems to contain uncommitted transactions. Please provide an explanation for the following:
create table t1
as
select *
from all_objects;
commit;
create table t2
as
select *
from all_objects;
update t2
set object_id = 1 where rownum = 1;
shutdown abort;
Neither table t2 nor the update to it were committed (right?), therefore, once the database starts up t2 should not be there. On the contrary, it is still there. In the Oracle Database Backup and Recovery Advanced Guide 10g Release 2, page 11-9, explains how the uncommitted transactions are removed (rolled back) in the Roll Backward step (transaction recovery) of the Instance recovery process.
Any insight on this is highly appreciated. Thanks.

Note also following scenario run with SYSDBA privileges:
bas002> select * from v$version;
BANNER
Oracle Database 10g Enterprise Edition Release 10.2.0.2.0 - Prod
PL/SQL Release 10.2.0.2.0 - Production
CORE    10.2.0.2.0      Production
TNS for 32-bit Windows: Version 10.2.0.2.0 - Production
NLSRTL Version 10.2.0.2.0 - Production
bas002>
bas002> drop table t2;
drop table t2
ERROR at line 1:
ORA-00942: table or view does not exist
bas002> create table t2
2 as
3 select *
4 from all_objects;
Table created.
bas002>
bas002> update t2
2 set object_id = 1 where object_id=258;
1 row updated.
bas002>
bas002> shutdown abort;
ORACLE instance shut down.
bas002> startup
ORACLE instance started.
Total System Global Area 192937984 bytes
Fixed Size                  1288484 bytes
Variable Size             130025180 bytes
Database Buffers           54525952 bytes
Redo Buffers                7098368 bytes
Database mounted.
Database opened.
bas002>
bas002> select count(*) from t2 where object_id=1;
COUNT(*)
         0
bas002>Message was edited by:
Pierre Forstmann

Need Instance Recovery?

I have a very strange situation right now:
1) Turn on Archive log Mode
2) Everything works fine ( both database & oracle apps)
3) Weekly full backup successful
4) Daily backup successful
5) Oracle databas hangs (shutdown & startup the database, same problem)
6) Turn off Archive log Mode
7) delete all archive logs
8) Everything works fine ( both database & oracle apps)
9) Try to turn on Archive log Mode again.
Failure: with message need to instance recovery???? How come it is? The Archive Log Mode is off and the database and oracle application works fine.
Any ideas? Or how to fix it?
FAN
Message was edited by:
user623471

Without really knowing the time gap and transaction volumes (ie archivelog rates) at these steps, and the exact message ...... possibly speculating ....
but I think that the most likely cause is that the ArchiveLog Destination filesystem is out of space for new archivelogs. That would cause
a. The "hang" at step 5 {because Oracle can't archive the active/current redolog}
b. The "hang" at startup in step 5 {because Oracle *still* cant archive ...}

Crash & instance recovery

what is the difference between crash recovery and instance recovery???

From http://download-uk.oracle.com/docs/cd/B10501_01/server.920/a96519/recov.htm#1004618
Crash recovery is used to recover from a failure either when a single-instance database crashes or all instances of an Oracle Real Application Clusters database crashes. Instance recovery refers to the case where a surviving instance recovers a failed instance in an Oracle Real Application Clusters database.

Crash recovery/ instance recovery

Hi,
How do oracle identifies crash recovery/ instance recovery
is required?
Regards,
Mathew

Hi,
>>But how do oracle identifies dabase is abnormally down and crash recovery is required?
I think that the checkpoint information that is desynchronized in redo log files and datafiles. It is necessary understand what is a checkpoint and what the CKPT process do. A checkpoint is a moment in time when all the changes (dirty blocks) made in the database buffer cache are made to the data files. The checkpoint is performed by the CKPT process and it creates an entry in the control file to identify the point in the online redo log file from where the instance recovery should begin in case of an instance failure. One of the ways a checkpoint is initiated is by the data block writer (DBWR) process. The DBWR process initiates a checkpoint by writing all modified data blocks in the data buffers (dirty buffers) to the data files. After a checkpoint is performed, all committed transactions are written to the data files. If the instance were to crash at this point, only new transactions that occurred after this checkpoint would need to be applied to the database to enable a complete recovery. Therefore, the checkpoint process determines which transactions from the redo logs need to be applied to the database in the event of a failure and subsequent recovery.
Cheers

Instance recovery needed

hi pls help me
while iam changing NOARCHIVELOG to ARCHIVELOG
iam getting at startup mount INSTANCE RECOVERY IS NEEDED
what should i do?
thank you

How did you make the database to archivelog mode?
SQL> CONNECT sys AS SYSDBA
SQL> STARTUP MOUNT;
SQL> ALTER DATABASE NOARCHIVELOG;
SQL> ALTER DATABASE OPEN;
or
Go to init parameter file
Set Archivelog parameters
log_archive_start=True
log_archive_format=%S.arc
log_Archive_dest=<Path for Archive log>
SQL> shutdown;
SQL> startup mount;
SQL> alter database archivelog;
SQL> alter database open;
SQL> select log_mode from v$database;
Thanks,
Cherrish

Smon performs the instance recovery if the smon also crash how the insta

smon performs the instance recovery if the smon also crash how the instance recovery performs ?
Thanks & Regards,
rehan

782191 wrote:
smon performs the instance recovery if the smon also crash how the instance recovery performs ?Rehan,
Instance recovery happens at the next instance startup , not when its currently running. If you kill SMON, you would kill the entire instance as well. As like Amardeep said, on the next startup, with SMON invoked, the isntance recovery would kick in.
HTH
Aman....

CheckPoint and Instance Recovery

hi,
what is checkpoint Exactly.How it will Preform Instance Recovery.
thanks
with Regards

hi,
thanks for reply.I read the documents and books,but i am not clear Exact point so that i have been posting here.
for example
semantic checking
Twq User a,b.Both of them has Emp tables but different Columns.
user A
select * from Emp;
User B
select * from Emp;
here i have doubts after B executing the query. how oracle take this, i think is it Semantic.
I am n't going to learn without Meaning a single word....
some terms not understanding so i am posting here
thanks
with regards

Undo behavior during instance recovery

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