[QUOTE][i]Originally posted by m_d_marian [/i]
[B]2. The redo log must be much more important because there are 2 files containing identical copy of another's data, why the rollback are less important?
[/B][/QUOTE]
You can't say which is more and which is less important for a database. You have to have both of them (plus a bunch of other things) to keep the database running. If one part of database is missing it will crash and/or you won't be able to start it again. Also, it is not true that you have to have 2 members of each redolog group. Oracle will gladly allow you to have only one copy of each redolog, but it is much more safe and recomended if you have them mirrored.

[QUOTE]
[B]3. The new value of data(redo log information) are written to database when commit or checkpoint occurs?
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Not necessarilly at commit time. DBWR writes dirty buffers only when it finds it necessary to do so. COMMITS do not trigger DBWR to write blocks to database files. But they do trigger LGWR to write changes from redo log buffer to redo log file on disk.

HTH,