B-tree is the most common data structure for indexes across different database systems. WiredTiger offers the option to use a Log-Structured Merge Tree instead of a B-tree for indexing.

An LSM tree can provide better performance when we have a workload of random inserts that would otherwise overflow our page cache and start paging in data from disk to keep our index up to date.

LSM indexes can be selected from the command line like this:

> mongod --wiredTigerIndexConfigString "type=lsm,block_compressor=zlib"

The preceding command chooses lsm index type and block_compressor zlib for indexes in this mongod instance.