It's my understanding that in the context of append-only skip lists, like what RocksDB uses for memtables, you can see wasted space at the end of a block within

It's my understanding that in the context of append-only skip lists, like what RocksDB uses for memtables, you can see wasted space at the end of a block within an arena but no fragmentation between records within a block. SSTables are another story.