Some Light on Roleback and commit transaction in sql recovery mode

If log truncation is performed when your transaction is still running then it will truncate only committed transactions and there is no impact on the current running transaction. if your transaction is completed and after that log truncation is performed it will clear your transaction. SQLServer internally maintain some lsn or modified pages details in one page so if you rebooted the server, if transaction is completed then changes applied into mdf files and there is no change in the DB and it will online after reboot. But if you rebooted the server in the middle of transaction then after server is online , database went into recovery mode and it will rollback all changes as per the changes after all the rollback completes DB will come into online.