MongoDB Documentation, Release 3.0.4 11.7 FAQ: MongoDB Storage 11.7.1 Storage Engine Fundamentals What is a storage engine? What will be the default storage engine going forward? 11.7.2 Can you mix storage engines in a replica set? 11.7.3 WiredTiger Storage Engine Can I upgrade an existing deployment to a WiredTiger? How much compression does WiredTiger provide? Yes. You can upgrade an existing deployment to WiredTiger while the deployment remains available by adding replica set members with the new storage engine and then removing members with the legacy storage engine. See the following sections of the Upgrade MongoDB to 3.0 (page 810) for the complete procedure that you can use to upgrade an existing deployment: The ratio of compressed data to uncompressed data depends on your data and the compression library used. By default, collection data in WiredTiger use Snappy block compression ; zlib compression is also available. Index data use prefix compression by default. Yes. You can have a replica set members that use different storage engines. When designing these multi-storage engine deployments consider the following: MMAPv1 is the default storage engine in 3.0. With multiple storage engines, you can decide which storage engine is best for your application. A storage engine is the part of a database that is responsible for managing how data is stored on disk. Many databases support multiple storage engines, where different engines perform better for specific workloads. For example, one stor- age engine might offer better performance for read-heavy workloads, and another might support a higher-throughput for write operations. This document addresses common questions regarding MongoDB’s storage system. If you don’t find the answer you’re looking for, check the complete list of FAQs (page 743) or post your question to the MongoDB User Mailing List$^{26}$. • the oplog on each member may need to be sized differently to account for differences in throughput between different storage engines. • recovery from backups may become more complex if your backup captures data files from MongoDB: you may need to maintain backups for each storage engine. • Change Replica Set Storage Engine to WiredTiger (page 812) • Change Sharded Cluster Storage Engine to WiredTiger (page 815) $^{26}$https://groups.google.com/forum/?fromgroups#!forum/mongodb-user 772 Chapter 11. Frequently Asked Questions