In-place update in MongoDB

There is a great new feature in the release note of MongoDB 3.5.12.

Faster In-place Updates in WiredTiger
This work brings improvements to in-place update workloads for users running the WiredTiger engine, especially for updates to large documents. Some workloads may see a reduction of up to 7x in disk utilization (from 24 MB/s to 3 MB/s) as well as a 20% improvement in throughput.

I thought wiredtiger has impeletementd the delta page feature introduced in the bw-tree paper, that is, writing pages that are deltas from previously written pages. But after I read the source code, I found it's a totally diffirent idea, in-place update only impacted the in-meomry and journal format, the on disk layout of data is not changed.

I will explain the core of the in-place update implementation.

MongoDB introduced mutable bson to descirbe document update as incremental(delta) update.

Mutable BSON provides classes to facilitate the manipulation of existing BSON objects or the construction of new BSON objects from scratch in an incremental fashion.

Suppose you have a very large document, see 1MB

{
   _id: ObjectId("59097118be4a61d87415cd15"),
   name: "ZhangYoudong",
   birthday: "xxxx",
   fightvalue: 100,
   xxx: .... // many other fields
}