Hi Florian,
TLOG backups in the SnapInfo folder are not deleted by the TLOG retention policy. It only deletes the SnapInfo Snapshots.
The reason for this is because deleting TLOG backups can lead to a state where up-to-the minute or Point-In-Time recovery can be hampered.
Think of a situation where you take 2 FULL backups and one of these fails verification. SMSQL will then retain both the FULL backups.
In this case you may need older TLOG backups.
Regards,
Sourav Chakraborty.
Technical Marketing Engineer (SMSQL , SMMOSS , VFM)
NetApp.