Lgwr busy process and the longer waits perform write
278 Part II ✦ Creating and Administering an Oracle Instance and Database
Checkpoints are covered in more detail in the section called “Checkpoints.”
An important thing to remember about redo log groups is that Oracle requires a minimum of two. If two are not available, the database cannot start. If the database is up and running and a group becomes unavailable, leaving only one, then the database closes.
Members from the same group should be on separate physical disks. This protects the database from a single point of failure for the group — the disk — and also helps reduce contention for writes by the LGWR. It is recommended that log files be placed on the fastest physical device on the system. LGWR is a busy process and the longer it waits to perform a write, the slower the database becomes. While this is true, it is uncommon to have different speed drives on the same system, there-fore, what is more important is placing the files on inactive drives, drives with little or no activity except for the log files. With this configuration, LGWR does not com-pete with other system IO requests. Members should also be stored on separate physical drives from the drives storing archive redo log files when running the database in ARCHVELOG mode. This helps avoid contention between the LGWR and ARCHIVER (ARCn) processes. Members should be placed on disks separate from data files. This eliminates contention between the LGWR and DATABASE WRITER (DBWn) process. This will also help prevent the loss of both data files and log files in the event of a disk failure.
Figure 7-1 illustrates the circular fashion by which online redo log groups are used, as well as an example of multiplexing online redo log members. Note that each group has two members (multiplexed), all groups are the same size, and members from the same group are on different physical disks.
| Group 1 | Group 2 | Group 3 | Group 4 | |
|---|---|---|---|---|
| Member 1a | Member 2a | Member 3a | Member 4a |
| Member 1b | Member 2b | Member 3b |
|---|
Creation of initial redo log groups and members
The initial redo log groups and their members are created when the database is cre-ated. This is a requirement. The location of the files (members) and the numbers assigned to the groups is part of the CREATE DATABASE syntax. You should note that it is possible to create multiple members per group at database creation time. This is highly recommended.
