standby 日志文件切换的问题

RFS[6]: Selected log<span style="color:#ff0000;"> 50</span> for thread 1 sequence 76106 dbid -2092952151 branch 856824811
Wed Mar 04 07:51:26 2015
Archived Log entry 76104 added for thread 1 sequence 76105 ID 0x8340c3a9 dest 1:
Wed Mar 04 07:56:08 2015
RFS[6]: Selected log <span style="color:#ff0000;">51</span> for thread 1 sequence 76107 dbid -2092952151 branch 856824811
Wed Mar 04 07:56:10 2015
Archived Log entry 76105 added for thread 1 sequence 76106 ID 0x8340c3a9 dest 1:
Wed Mar 04 08:00:25 2015
RFS[6]: Selected log <span style="color:#ff0000;">50</span> for thread 1 sequence 76108 dbid -2092952151 branch 856824811
Wed Mar 04 08:00:27 2015
Archived Log entry 76106 added for thread 1 sequence 76107 ID 0x8340c3a9 dest 1:
Wed Mar 04 08:04:38 2015
RFS[6]: Selected log<span style="color:#ff0000;"> 51</span> for thread 1 sequence 76109 dbid -2092952151 branch 856824811
Wed Mar 04 08:04:40 2015
Archived Log entry 76107 added for thread 1 sequence 76108 ID 0x8340c3a9 dest 1:

以上是一个DG环境中物理备库的日志，为什么standby 的日志文件只在两组日志文件中切换，难道只有两组standby 日志文件

SQL>  select GROUP#,SEQUENCE#,USED,ARCHIVED,STATUS from v$standby_log;

GROUP#  SEQUENCE#       USED ARC STATUS
---------- ---------- ---------- --- ----------
50          0          0 NO  UNASSIGNED
51      76111   90693120 YES ACTIVE
52          0          0 NO  UNASSIGNED
53          0          0 NO  UNASSIGNED
54          0          0 NO  UNASSIGNED
55          0          0 NO  UNASSIGNED
56          0          0 NO  UNASSIGNED
57          0          0 NO  UNASSIGNED
58          0          0 NO  UNASSIGNED
59          0          0 NO  UNASSIGNED
60          0          0 NO  UNASSIGNED

GROUP#  SEQUENCE#       USED ARC STATUS
---------- ---------- ---------- --- ----------
61          0          0 NO  UNASSIGNED
62          0          0 NO  UNASSIGNED
63          0          0 YES UNASSIGNED
64          0          0 YES UNASSIGNED
65          0          0 YES UNASSIGNED
66          0          0 YES UNASSIGNED
67          0          0 YES UNASSIGNED
68          0          0 YES UNASSIGNED

从以上信息中可以看出，不止两组standby 日志组，那为什么只在两组日志文件中进行切换，我的猜测是：以这个例子为例，首先备库使用组50接收主库的redo，当组50写满后切换到组51，当组51写满后，组50的standby 日志文件已经应用完，并且已归档，所以可以继续使用组50，这跟redo 日志的文件的使用方法不太一样。