What is WiredTiger? What is MMAPv1?
WiredTiger is MongoDB’s new storage engine. It is available as an option for all 64-bit MongoDB 3.0 and higher builds. Among other features, it supports document-level locking and compression on disk. Check out MongoDB’s docs on WiredTiger for more information.
MMAPv1 is MongoDB’s traditional memory-mapped files storage engine. In MongoDB 3.0 we added collection-level locking while remaining compatible with the on-disk storage of MongoDB 2.6 and older.
Why you should match
Matching your currently running storage engine to your backed-up engine is critical to easy restores. If your restore files are in a different format than what you are used to running, you will have to make certain that you set your command line options correctly, and differently from what you are running elsewhere.
Where to change your setting
First, you have to be running the new MMS. MMS Classic customers cannot change their backup format, and only have MMAPv1 backups available to them. You are running the new MMS if the upper-left of your MMS window looks like this:
Once you are in the new MMS, you can change your backup format by going to the “Backup” tab, click the “…” for your replica set or cluster and choose “Edit Storage Engine”
Once there, you can choose MMAPv1 (“MongoDB Memory Mapped Files”) or WiredTiger:
Once you make this change, an initial sync will be triggered. Choose the server you want to sync from and confirm. An initial sync is required so we can build your new backup. This will not change your existing snapshot formats, so if you request an older restore, it will still be in MMAPv1. You can tell if a snapshot is in WiredTiger format by looking at the “Mongod Version” column on your snapshots listing page (just click on a replica set name on your Backup tab). If the version has “(wiredTiger)” after it, the snapshot is in WiredTiger format. You can see I converted this replica set to WiredTiger: