Shrinking The Log File On A DB That Is Mirrored

Hello,

I'm having couple of DB that are mirrored.

my concerne is regarding the Log file size.

I'm running the following steps:

BACKUP DATABASE [DBName] TO DISK = N'Backup_File' WITH NOFORMAT, INIT, SKIP, NOREWIND, NOUNLOAD, STATS = 10

then

BACKUP LOG [DBName] TO DISK = N'Backup_File' WITH NOFORMAT, NOINIT, SKIP, NOREWIND, NOUNLOAD, STATS = 10

if I try to run a SHRINKFILE (DBName_log, 20)

I receive the following info:

Cannot shrink log file 2 (DBName_log) because all logical log files are in use

I'n a bit tense using a

BACKUP LOG [DBName] WITH TRUNCATE_ONLY

as it's part of a mirroring.

but I need to reduce the size of the log file.

thank four your suggestions

Eric