Track separately:

In #3 @deuter0n wrote:

how much overhead does each btree node takes ? I tried to run an unrealistic case: saving 1M empty v, and the db size is ~60MB

for _, i := range rand.Perm(1<<20) {
    k := make([]byte, 4)
    binary.BigEndian.PutUint32(k, uint32(i))
    if e = btree.Set(k, nil); e != nil {
        log.Fatal(e)
    }
}

Investigate if it's a storage space (leak) bug or if the implementation is just crappy. Fix/improve it iff possible w/o breaking backward compatibility with any existing DBs.