synchronous replication + fsync=off?

Hello,

I have two servers with battery backed power supply (USV). So it is unlikely, that both will crash at the same time.

Will synchronous replication work with fsync=off?
That means we will commit to system cache, but not to disk. Data will not survive a system crash but the second system should still be consistent.

Or: will it work with
master: fsync=off
and
slave(s): fsync=on?
In this case master is free for read-queries and slave has time to do all the heavy writing. Data on master will not survive a crash but can be restored from slave.

Thanks,

Joerg