If you are running asynchronous database mirroring, then there could be a backlog of transaction log records that have not been sent from the principal to the mirror (called the database mirroring SEND queue). The transaction log records cannot be freed until they have been successfully sent. With a high rate of transaction log record generation and limited bandwidth on the network (or other hardware issues), the backlog can grow quite large and cause the transaction log to grow.
On the mirrored database, you cannot backup the log file with TRUNCATE_ONLY. Here the steps to shrink the log file for a database participating in mirroring
- Backup the log file to a location
BACKUP Log YourDatabaseName TO DISK = 'D:\BACKUP\DBNAME_20090201.TRN'
- Check if there is enough free space on perform the shrink operation
SELECT name ,size/128.0 - CAST(FILEPROPERTY(name, 'SpaceUsed') AS int)/128.0 AS AvailableSpaceInMB
If there is no sufficient free space then the shrink operation cannot reduce file size.
- Check if all the transactions are written into the disk
The status of the last transaction should be 0. If not, then backup the transaction log once again.
- Shrink the log file
DBCC SHRINKFILE(logfilename , target_size)
If the transaction lof file does not shrink after performing the above steps then backup the log file again to make more of the virtual log files inactive.
Also check the column LOG_REUSE_WAIT_DESC in the sys.databases catalog view to check if the reuse of the transaction log space is waiting on anything.
Check this link to find the factors that can delay log truncation