For LevelDB, you can either increase the batch size to increase the
number of items written per fsync call, or you can loosen the durability
guarantees you get. Ultimately, whenever you flush to disk durably, you
will wait for the disk to write and performance will be worse than when
you don't have to wait for the disk.
You can improve performance by setting the "relatime,data=writeback"
filesystem options on ext4. This will change the guarantees made by the
filesystem in a way that can impact some applications, but should not
mean anything negative for LevelDB.
-Robert