Some background porcess question

Question

Hi~
I have some confuse question about background process.
1.When I set ALTER SYSTEM CHECKPOINT,all dirty buffers are written,is it include non commit transaction entry??
2.When there is 1MB of redo log buffer,LGWR writes redo entries to the redo log files,is it include non commit transaction redo ?
when a datafile damaged,we restore it from backup,and roll forward from all archive log and online redo log file,how did oracle know what data need rollback? is the undo segment will keep these uncommit transaction so it know who need rollback?

Answer 1

Shu shic chun -
1.
When a buffer in the database buffer cache is modified, it is marked dirty. A cold buffer is a buffer that has not been recently used according to the least recently used (LRU) algorithm. The DBWn process writes cold, dirty buffers to disk so that user processes are able to find cold, clean buffers that can be used to read new blocks into the cache
2.Sometimes, if more buffer space is needed, LGWR writes redo log entries before a transaction is committed.
These entries become permanent only if the transaction is later committed.
After rolling forward, the data blocks contain all committed changes. They may also contain uncommitted changes that were either saved to the datafiles before the failure, or were recorded in the redo log and introduced during cache recovery.
Undo blocks (whether in rollback segments or automatic undo tablespaces) record database actions that should be undone during certain database operations. In database recovery, the undo blocks roll back the effects of uncommitted transactions previously applied by the rolling forward phase.
After the roll forward, any changes that were not committed must be undone. Oracle applies undo blocks to roll back uncommitted changes in data blocks that were either written before the crash or introduced by redo application during cache recovery. This process is called rolling back or transaction recovery.