https://webassets.mongodb.com/_com_assets/collateral/MongoDB-Performance-Best-Practices.pdf has the following:

Readahead size should be set to 32. Use the
blockdev --setra <value> command to set the
readahead block size to 32 (16kb) or the size of most
documents, whichever is larger.
If the readahead size is much larger than the size of the
data requested, a larger block will be read from disk – this
is wasteful as most disk I/O in MongoDB is random. This
has two undesirable consequences:
8
1. The size of the read will consume RAM unnecessarily.
2. More time will be spent reading data than is necessary.
Both will negatively affect the performance of your
MongoDB system. The readahead block size should not
be set lower than 32.

Note: "The readahead block size should not be set lower than 32."

This contradicts our production notes, which recommend 16 or 0 and are themselves the subject of DOCS-9158, which calls for a change to 0 only.

The white paper should be corrected to recommend a setting of 0 for WT.