title: How many bytes occupied by a member in Redis Set
body: <p>I am using Redis as a in-memory hashset. After I insert 1M 8-byte keys (in binary) into a Set, I find Redis USED_MEMORY comes to about 100M, which means a single member takes 100 bytes? why ?</p>
  
  <p>Or how can I conf Redis to save it memory usage.</p>