lgwr什么时候写重做项到online logfile

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Log Writer Process (LGWR)The log writer process (LGWR) manages the redo log buffer. LGWR writes one contiguous portion of the buffer to the online redo log. By separating the tasks of modifying database buffers, performing scattered writes of dirty buffers to disk, and performing fast sequential writes of redo to disk, the database improves performance.
In the following circumstances, LGWR writes all redo entries that have been copied into the buffer since the last time it wrote:

• A user commits a transaction (see "Committing Transactions").
  
• An online redo log switch occurs.
  
• Three seconds have passed since LGWR last wrote.
  
• The redo log buffer is one-third full or contains 1 MB of buffered data.
  
• DBWn must write modified buffers to disk.
  Before DBWn can write a dirty buffer, redo records associated with changes to the buffer must be written to disk (the write-ahead protocol). If DBWn finds that some redo records have not been written, it signals LGWR to write the records to disk and waits for LGWR to complete before writing the data buffers to disk.