How expensive is semi-synchronous replication? (no replies)

Hi,

Planning to just have 2 servers.

Initially though master-slave with asynchronous replication. However, unsure of possible data loss if master crashes and we manually failover to slave. Can anyone comment on protecting ourselves in this situation, even if it's manual steps?

Now wondering about using semi-synchronous replication. How significant is the performance overhead if we make this switch? Is data loss guaranteed eliminated if the master crashes and we failover to the slave?

thanks in advance!