| shikhu |
Posted: Jul 05, 2007 10:05:05 AM |
Total Post: 61
Joined: Jul, 2006
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Hi All,
I want to know what exactly happens internally when we issue a commit, means How Oracle going to know that what data is committed and what is uncommited as my data files may contain both types of data. |
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 Ratnaker |
| Posted: Jul 06, 2007 09:10:09 AM |  |
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Total Post: 136
Joined: Apr, 2007
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Hi Sikku,
To commit a transaction makes permanent the changes resulting from all SQL statements in the transaction. The changes made by the SQL statements of a transaction become visible to other user sessions' transactions that start only after the transaction is committed.
Ratnaker
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| shikhu |
| Posted: Jul 06, 2007 09:55:03 AM | |
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Total Post: 61
Joined: Jul, 2006
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hi KP,
Question is : when a transaction is going on and is not commited and there is a logswitch then this uncommited tranx. is written to datafiles, meaning to say that my datafile may contain commited and uncommited data and suddenly my instance got crashed then SMON activates and rollback that incompleted/uncommited transaction from Undo t/s , my confusion is howcome SMON come to know that this transaction is uncommited or what shud I say an open transaction , and need a rollback.
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| Ratnaker |
| Posted: Jul 09, 2007 01:54:46 AM | |
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Total Post: 136
Joined: Apr, 2007
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Hi shikku,
The goal of crash and instance recovery is to restore the data block changes located in the cache of the terminated instance and to close the redo thread that was left open. Instance and crash recovery use only online redo log files and current online datafiles. Oracle recovers the redo threads of the terminated instances together.
Crash and instance recovery involve two distinct operations: rolling forward the current, online datafiles by applying both committed and uncommitted transactions contained in online redo records, and then rolling back changes made in uncommitted transactions to their original state.
Crash and instance recovery have the following shared characteristics:
Redo the changes using the current online datafiles (as left on disk after the failure or SHUTDOWN ABORT)
Use only the online redo logs and never require the use of the archived logs
Have a recovery time governed by the number of terminated instances, amount of redo generated in each terminated redo thread since the last checkpoint, and by user-configurable factors such as the number and size of redo log files, checkpoint frequency, and the parallel recovery setting
Oracle performs this recovery automatically on two occasions:
At the first database open after the failure of a single-instance database or all instances of an Oracle Real Applications Cluster database (crash recovery).
When some but not all instances of an Oracle Real Application Clusters configuration fail (instance recovery). The recovery is performed automatically by a surviving instance in the configuration.
The important point is that in both crash and instance recovery, Oracle applies the redo automatically: no user intervention is required to supply redo logs.
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| positive |
| Posted: Jun 29, 2008 08:56:17 PM | |
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Total Post: 361
Joined: Jun, 2008
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when commit is issued,
data from db buffer cache is written to disk and header is updated.simultaneously scn is updated in controlfile.data from redolog buffer are written to redo logs.
To get more details visit:
http://neworacledba.blogspot.com/
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