ARCHIVE_LAG_TARGET手動設定歸檔間隔時間，強制日誌切換

          在Standby中，有時候我們為了減少資料損失，可以設定ARCHIVE_LAG_TARGET 引數來進行強制日誌切換 。   一般設定 1800秒 ~ 7200s 之間，過低會導致效能問題，過高對於強制日誌切換就沒有意義了(可能還沒有到時間就因為online redo log 滿了自動切換了) 。

Parameter type	Integer
Default value	0 (disabled)
Parameter class	Dynamic: ALTER SYSTEM
Range of values	0 or any integer in [60, 7200]
Real Application Clusters	Multiple instances should use the same value.

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