Database Reference
In-Depth Information
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i highly recommend that you multiplex your online redo log groups and, if possible, have each member on a
separate physical device governed by a separate controller.
Tip
The log writer is the background process responsible for writing transaction information from the redo log buffer
(in the SGA) to the online redo log files (on disk). Log writer flushes the contents of the redo log buffer when any of the
following are true:
COMMIT
or
ROLLBACK
issued.
•
A
•
A log switch occurs.
•
Three seconds go by.
•
The redo log buffer is one-third full.
•
The redo log buffer fills to one megabyte.
The online redo log group that the log writer is actively writing to is the
current online redo log group
. The log
writer writes simultaneously to all members of a redo log group. The log writer needs to successfully write to only one
member in order for the database to continue operating. The database ceases operating if the log writer can't write
successfully to at least one member of the current group.
When the current online redo log group fills up, a log switch occurs, and the log writer starts writing to the next
online redo log group. The log writer writes to the online redo log groups in a round-robin fashion. Because you have
a finite number of online redo log groups, eventually the contents of each online redo log group are overwritten.
If you want to save a history of the transaction information, you must place your database in archivelog mode (see the
section “Implementing Archivelog Mode,” later in this chapter).
When your database is in archivelog mode, after every log switch the archiver background process copies the
contents of the online redo log file to an archived redo log file. In the event of a failure the archived redo log files allow
you to restore the complete history of transactions that have occurred since your last database backup.
Figure
2-1
displays a typical setup for the online redo log files. This figure shows three online redo log groups,
each containing two members. The database is in archivelog mode. In the figure, group 2 has recently been filled with
transactions, a log switch has occurred, and the log writer is now writing to group 3. The archiver process is copying
the contents of group 2 to an archived redo log file. When group 3 fills up, another log switch will occur, and the log
writer will begin writing to group 1. At the same time, the archiver process will copy the contents of group 3 to archive
log sequence 3 (and so forth).
