DBWn and LGWR writes

DBWn writes when....

1. Server process takes too long to find a free buffer (too long is determined by internal parameters).
2. checkpoint queue (list of ditry buffers waiting to be written to the disk) becomes too long.
3. Three second time out. (Every three second DBW will writer some dirty buffers to disk).
4. alter system checkpoint is called. all dirty buffers are written to the disk.

checkpoints automatically occur when

1. tablespace is dropped
2. tablespace is made read only
3. tablespace is put into hot backup mode
4. tablespace is taken offline

LGWR writes when....

1. User commits a transaction
2. log buffer is one third full
3. Just before DBWn writes
4. Three second time out