[slver]

Its document model is vanilla BSON on disk.

[avinassh]

It possible to update the BSON without loading the entire document in the memory? I am curious how Mongo does updates.

[slver]

It’s a memory mapped file. If you update a bool or number value it’ll rewrite just that. If you update anything else it’ll often have to rewrite the entire document due to variable size.

[MaBeuLux88]

That was true with MMAPv1. It's not since March 2015, when WiredTiger was introduced with MongoDB 3.0.

[the-dude]

I assume when you say just that, it will write an entire page.

[slver]

In practice yeah I guess.

But rewriting the document is more extensive, it might even have to be moved (because Mongo allocates certain number of bytes for it, and it might outgrow that).