日期：2016年3月8日

以下摘自Oracle log files : An introduction：

Online redo Oracle log files are filled with redo records. A redo record, also called a redo entry, is made up of a group of change vectors, each of which is a description of a change made to a single block in the database.

Redo日志文件由redo record组成，每个redo record（又称作redo entry）又由一组change vector构成。

The online redo log file that Log Writer (LGWR) is actively writing to is called the current online redo log file. Online redo Oracle log files that are required for instance recovery are called active online redo log files. Online redo log files that are not required for instance recovery are called inactive.

参考下面例子：

SQL> select group#,members,bytes/1024/1024,status from v$log;

    GROUP#    MEMBERS BYTES/1024/1024 STATUS
---------- ---------- --------------- ----------------
         1          1             100 INACTIVE
         2          1             100 ACTIVE
         3          1             100 INACTIVE
         4          1             300 CURRENT

CURRENT是LGWR进程正在写入的文件；ACTIVE是用于recovery的；INACTIVE则不是。

以下摘自Oracle 12c For Dummies：

The checkpoint process(CKPT) is responsible for initiating check points. A check point is when the system periodically dumps all the dirty buffers to disk. Most commonly, this occurs when the database receives a shutdown command. It also updates the data file headers and the control files with the check point information so the SMON know where to start recovery in the event of a system crash.

CKPT进程是初始化checkpoint，而checkpoint的作用是让系统可以周期性地把“脏数据”更新到磁盘。