archive_lag_target参数

需求,由于一套生产环境归档日志切换频率过低,建议修改参数,使其间隔一定时间周期自动切换生成归档日志;
SQL>select thread#,sequence#,to_char(completion_time,'yyyy-mm-dd hh24:mi') from v$archived_log where completion_time>sysdate-;
    THREAD#  SEQUENCE# TO_CHAR(COMPLETION_TIME,'YYYY-MM
---------- ---------- --------------------------------
                  -- :
                  -- :
                  -- :
                  -- :
测试环境归档日志切换频率非常低,生产环境类似，如果生产环境online redo出现问题，将丢失数十小时的数据。
SQL>select a.ksppinm,b.ksppstvl,a.ksppdesc from x$ksppi a,
x$ksppcv b where (a.indx=b.indx) and a.ksppinm like '%archive_lag_target%';
--------------------------------------------------------------------------------
archive_lag_target        Maximum number of seconds of redos the standby could lose  秒为单位,重做日志切换时间

生产环境建议配置1800s,即建议30分钟日志切换一次。

How to force a log switch (文档 ID 2320185.1)
There are cases when your DB activity is not significant so the number of redo log switches is very small or even 0 for certain timeframes. In these conditions, how do we force a periodical log file switch?

Setup of archive_lag_target anywhere between [60, 7200] seconds:

use of setting archive_lag_target in non-standby envrionment

ARCHIVE_LAG_TARGET limits the amount of data that can be lost and effectively increases the availability of the standby database by forcing a log switch after the specified amount of time elapses.

A 0 value disables the time-based thread advance feature; otherwise, the value represents the number of seconds. Values larger than 7200 seconds are not of much use in maintaining a reasonable lag in the standby database. The typical, or recommended value is 1800 (30 minutes). Extremely low values can result in frequent log switches, which could degrade performance; such values can also make the archiver process too busy to archive the continuously generated logs.

SQL> alter system set archive_lag_target=; --15分钟

SQL>  select sysdate from dual;
SYSDATE
-------------------
-- ::

测试环境没有数据,按照参数,应该是再03:52分,alert应该存在日志切换记录信息。
Alert日志信息

Mon Aug  ::
Thread  advanced to log sequence  (LGWR switch)
  Current log#  seq#  mem# : /11.2.0.4/app/oracle/oradata/tt11204/redo03.log
Mon Aug  ::
Archived Log entry  added for thread  sequence  ID 0x756770e0 dest :
Mon Aug  :: 2019