Oracle Tips by Burleson

Number and Placement of Redo Logs

Oracle requires at least two groups of one redo log. If you are archiving, three are suggested. In a number of installations, up to six or more have been defined. If you do a lot of update activity and have numerous users, more than six may be required. When a log fills, the next one in the queue is opened, and the previously active log is marked for archive (if you have archiving enabled). The logs are archived on a first-in, first-out basis, so, depending on the speed that the log groups can be written to disk or tape, more than one log group may be waiting to be archived. One redo log group is used at a time, with multiple users writing into it at the same time. The size of the redo logs in a group depends on one critical piece of data: How much data can you afford to lose on a system crash?

You see, the smaller the log group size, the more often it is written to disk and the less data (time-wise) is lost. The larger the log group size, the less often it is written to disk and the more data (time-wise) is lost. For instance, if your log groups are filling every 10 minutes, then you may lose 10 minutes’ worth of data should the disk(s) crash that holds that redo log group’s files. It has been demonstrated on an active system that a 100-MB redo log group may only last a few seconds. In an inactive or read-only-type situation, a 100-MB redo log may last for hours. It is all dependent on how the database is being used and the size of the redo log group. Remember, a group of three 100-MB redo logs is actually treated as only a single 100-MB redo log (the other two files are mirrors). If you mirror redo logs by placing the group members on separate disks (not just on separate file systems; be sure it is separate physical disks), then your ability to recover from a disk array or controller crash increases manyfold.   

You have to balance the needs for restoration and minimal data loss against time to recover data. Obviously, if you have archiving happening every minute and your normal workday is eight hours, you will have 480 logs written to disk daily. Over a five-day workweek, this turns into 2,400 files. If you have to restore from a crash or other disaster, you may have to apply all of these to your last backup to bring the database current to the time of the crash. In one case, a Remote DBA had to apply 9,000-plus files to recover his system because he hadn’t looked at how often his redo logs were archiving. Needless to say, he pays more attention now. The minimum size for redo log groups is 50 KB. By default the Oracle9i DBCA creates 100 megabyte redo log.

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