Files in a WiredTiger database

The WiredTiger storage engine

WiredTiger is a high performance, key-value storage engine that is the default storage engine in MongoDB 3.2 and newer.

In a MongoDB deployment using the WiredTiger engine, the storage.dbpath directory is managed by WiredTiger. That is, WiredTiger needs the content of the directory as a whole to function properly. Removing files from this directory could render WiredTiger (and thus MongoDB) unable to start. A notable exception is the diagnostic.data directory, which contains non-personally identifiable information regarding the status of the database for troubleshooting purposes.

Files in a WiredTiger database

A newly created MongoDB 3.2 database started with the default settings typically consists of:

WiredTiger
WiredTigerLAS.wt
WiredTiger.lock
WiredTiger.turtle
WiredTiger.wt
_mdb_catalog.wt
mongod.lock
sizeStorer.wt
collection-0-601483983079373625.wt
index-1-601483983079373625.wt
diagnostic.data
storage.bson

Important

Do not remove or modify any of these files. Doing so may leave the database in an inoperable state.

Descriptions of each of the files and its functions are as follows:

• WiredTiger: the WiredTiger version file
  This text file contains the version of WiredTiger used to create the database, and is used by WiredTiger on startup to determine whether a database is present.
• WiredTiger.wt: the WiredTiger metadata file
  The main metadata file that tracks information about which collections and indexes exist in the database, as well as the most recent durable update (checkpoint) for each of them.
• WiredTiger.turtle: engine configuration file for the WiredTiger.wt file
  This text file contains the WiredTiger engine configuration for the WiredTiger.wt metadata file described above.
• WiredTigerLAS.wt: the WiredTiger lookaside file
  MongoDB configures WiredTiger in-memory cache to contain no more than a specified percentage of dirty data (defaults to 20% in MongoDB 3.2.10). If the dirty data exceeds the configured number and WiredTiger is required to keep the dirty data for operational reasons, this file contains the disk spill of the dirty data.
• _mdb_catalog.wt: the MongoDB metadata file
  This file contains the metadata and catalog of MongoDB databases, collections, and indexes, including which WiredTiger data file maps to each MongoDB collection or index.
• WiredTiger.lock and mongod.lock: lock files
  MongoDB and WiredTiger use these lock files to ensure that only a single process accesses the database at any particular time.
• sizeStorer.wt: the MongoDB collection size tracking file
  This file contains information about the size and document count of each MongoDB collection.
• collection-X-XXX.wt and index-X-XXX.wt: MongoDB collection and index files
  Each MongoDB collection is stored in a collection-X-XXX.wt file, and each MongoDB index is stored in a index-X-XXX.wt file. The X-XXXportion of the filename represents a random number used by WiredTiger as a unique internal identifier. If MongoDB is started with the --directoryPerDB option these files may appear in a subdirectory of the dbpath.

Files created by optional settings

• journal directory: WiredTiger journal files
  If a database is started without the --nojournal option, the database directory also contain a journal subdirectory that has the following contents:

  WiredTigerLog.0000000001
  WiredTigerPreplog.0000000001
  WiredTigerPreplog.0000000002
  

  Of importance here is the WiredTigerLog.XX file, which contains the Write Ahead Log (WAL) maintained by WiredTiger.