is there a way to replicate mysql while the master server already has a lot of data.I tried the normal way, but I had difficulty getting the MASTER_LOG_POS value. how can the slave server be able to replicate data that previously existed on the master server.
Generally you start with an exact full copy of your existing database. This means creating a real copy of your MySQL data directory (while the server is off), go with a (consistent) snapshot, or use a tool like Percona XtraBackup.
Only after you have 2 identical MySQL servers, you can start replicating. Note that using a tool like mysqldump is not a good idea for consistent snapshots.