Description

The docs could be clearer when handling the phrase "jumbo chunks". This is used interchangably to refer to each of:

1. chunk that has been marked with the jumbo: true flag
2. chunk that is indivisible, and will be marked as jumbo: true if the balancer attempts to move it (but currently doesn't have jumbo: true)
3. chunk that is divisible, but has been marked as jumbo: true for some other reason (eg. SERVER-19919, SERVER-21931).
4. chunk that is informally very large and difficult to migrate or work with (but may in fact be divisible)

I'm not sure if #3 and #4 are actually a problem in the docs — it might be more in discussions elsewhere about jumbo chunks.

In any case, the docs would benefit from some discussion around "jumbo as in jumbo: true" vs "jumbo as in indivisible and large".

Examples:

• https://docs.mongodb.com/manual/core/sharding-data-partitioning/#jumbo-chunks says "In some cases, chunks can grow beyond the specified chunk size but cannot undergo a split. The most common scenario is when a chunk represents a single shard key value. Since the chunk cannot split, it continues to grow beyond the chunk size, becoming a jumbo chunk."
• https://docs.mongodb.com/manual/core/sharding-data-partitioning/index.html says "Automatic splitting only occurs during inserts or updates." and "Inserts and updates may trigger splits." and later on that the balancer just just moves the chunks around that were split during insert or update to regain an even distribution amongst the shards.

Scope of changes

Impact to Other Docs

MVP (Work and Date)

Resources (Scope or Design Docs, Invision, etc.)