it writes the uncommited and the comited data from the
redolog buffer to the redolog file

when one third of redo log buffer is filled.
when transcation is completed.
time out of 3 sec.

1.Every 3 secs
2.At commit(when a tx commits)
3.When one third of redo log buffer is filled.
4.When there is more than 1 Mb of redo entries(redo data or
changes) in the redo log buffer
5.Before DBWR background process writes dirty buffers from
db cache to datafiles(database)

At any of the above instances(cases) lgwr writes the redo
entries or redo data or changes from the redo log buffer to
the redo log file.