Step1.将DB脱离可用性组
Step2.修改为简单恢复模式>收缩文件>修改回完整恢复模式

-- Truncate the log by changing the database recovery model to SIMPLE.
ALTER DATABASE [DB]
SET RECOVERY SIMPLE;
GO
-- Shrink the truncated log file to 1 MB.
DBCC SHRINKFILE (DB_Log, 100);
GO
-- Reset the database recovery model.
ALTER DATABASE [DB]
SET RECOVERY FULL;
GO

Step3.完整备份
Step4.将DB重新加入可用性组