Adding Transaction logging Issue

We are planning to add transaction logging to a large existing application... We use standard engine and on-line engine We are soliciting any advice/guidance on these topics: 1) Database/Administrative Aspects Best approach for when/how to clear transaction log files ? Can it be automatic ? Any suggestions on backup/recovery techniques ? 2) Performance Issues Are there significant perfromance hits with transactions? 3) Locking We need row level locking ....with large numbers of records in files When/how does Informix promote row level locking to table level locking? Pessimistic versus Optimistic Locking techniques in programs ? 4) Purge/Update programs Any special techniques needed for writing large purge/update routines where 100,000 records exist that need to be purged/updated Any responses/guidance would be appreciated !! Thank you