MongoDB wiredTiger storage engine uses both WiredTiger INTERNAL cache and FILE system cache for storing data. If we do not define wiredTiger internal cache by default it utilizes larger of either 256MB or 50% of (RAM -  1GB). For example, if a system has a total 0f 6GB RAM, so 2GB (50% 0f 6GB – 1 GB) will be allocated to wiredTiger internal cache. This default setting assumes that there is only one mongod process running. In case we have multiple mongodb instances on the server we should decrease the wiredTiger internal cache size to accommodate other instances.

WiredTiger also provides compression options for both collections and indexes by default. While snappy compression is used for collections, prefix compression is used for all indexes. We can set the compression at the database as well as collection and index level.

 WiredTiger internal cache and filesystem cache differs in terms of data REPRESENTATION from on-disk format.

1. Data is stored in the same manner for filesystem cache as the on-disk format, including the data files compression. The operating system uses the filesystem cache to reduce disk i/o.
2. Although indexes are CREATED in different representation in wiredTiger internal cache than on-disk format, they still take advantage of index prefix compression to reduce RAM.
3. The collected data in the WiredTiger internal cache uses different representation than the on-disk format. This data is uncompressed which allows it to be manipulated by the server. While on-disk format uses block compression which provides significant storage savings.

All free memory that is not used by wiredTiger cache or by any other process is automatically used by MongoDB filesystem cache.