1) redo rollback for redo and undo slave wires are protected by redo, so that undo is obtained in buffercache after the rollback, and apply the undo to the data blocks and index blocks in buffercache, so that those data files can catch up with the rest of the database, so as to maintain database consistency. The two are bridges between DBAs and developers.
1) redo rollback for redo undo is protected by redo to get undo in buffer cache after the rollback, and apply the undo to the data blocks and index blocks in the buffer cache, so that those data files can catch up with the rest of the database, so as to maintain database consistency. The two are bridges between DBAs and developers.
1) redo and undo's "broken wires"
Roll back redo
Roll back undo
Undo is protected by redo, so that undo can be obtained in the buffer cache after the rollback, And the undo is applied to the data blocks and index blocks in the buffer cache, this allows data files to "catch up with" the rest of the database to maintain database consistency.
The two are bridges between DBAs and developers.
The rollback process does not involve redo logs. Only the redo logs are read during recovery and archiving. Because the redo log is used for writing, not for reading!
2) Notes for deleting online redo log file groups
① When the log group is active or current, it cannot be deleted.
② After a log group is deleted at the database level, files on the operating system will not be deleted by the level chain.
③ An Oracle database instance must contain at least two online redo log groups.
3) who writes the redo buffer?
> Does redo buffer write lgwr?
No. Server (shadow) processes write to log buffer. LGWR writes redo records from the log buffer to the log file.
> Is the undo buffer read by DBWR because it is in the db cache buffer?
Not quite sure what you mean. "Undo buffer" is just a buffer in buffer cache that corresponds to a block in an undo tablespace.
Treat it like any other data block. It's written into buffer cache by a server process and written out to datafiles by DBWn.
> IO (especially asynchronous) is involved, and a special process is required to complete the process.
A server process can always read datafiles. But normally only DBWn can write to them. Also remember a server process can read and write a tempfile
