I've seen many innovations on rocksdb based on leveldb.
But, there is a hole we can do to improve.

As in skiplist, the internal key packed as:
{sequence|type|user_key|value}

if one key in skiplist is:

{100|0x01|'key1'|'val1'} -->node0

then, we insert same key with different value:

{101|0x01|'key1'|'val2'} -->node1

So, the same key is in two nodes, when we fall in the 'hot' function 'DBIter::FindNextUserEntryInternal', we have to do as follows:

if (ParseKey(&ikey) && ikey.sequence <= sequence_) {
      if (skipping &&
          user_comparator_->Compare(ikey.user_key, saved_key_.GetKey()) <= 0) {

if we changed the node structure from:

struct node {
   Key key;
}

to

struct node {
   Key keys[];
}

We don't need to do user_comparator_->Compare.
The 'iter->next' means that i am a 'restart'&'difference' key.

The OK version is here:
https://github.com/shuttler/nessDB/blob/master/engine/skiplist.h
https://github.com/shuttler/nessDB/blob/master/engine/skiplist.c

-BohuTANG