fsync() is a blocking call, which requires confirmation from the disk before your process continues. roughly, this means that redis can't do anything while the fsync() happens. while this isn't any more work (cpu) it is significantly slower because of all of the time spent waiting.
Hmm . . . as long as the requests are not being handled on the same thread as the one doing the fsync, this shouldn't stop work in the process or make anything appreciably slower.
