MongoDB Documentation, Release 3.0.4 When the primary has stepped down, shut down its instance and upgrade by replacing the mongod binary with the 2.0.x binary. A compact command is now available for compacting a single collection and its indexes. Previously, the only way to compact was to repair the entire database. When going to disk, the server will yield the write lock when writing data that is not likely to be in memory. The initial implementation of this feature now exists: See SERVER-2563 883 for more information. The specific operations yield in 2.0 are: MongoDB 2.0 reduces the default stack size. This change can reduce total memory usage when there are many (e.g., 1000+) client connections, as there is a thread per connection. While portions of a thread’s stack can be swapped out if unused, some operating systems do this slowly enough that it might be an issue. The default stack size is lesser of the system setting or 1MB. v2.0 includes significant improvements to the index (page 523). Indexes are often 25% smaller and 25% faster (depends on the use case). When upgrading from previous versions, the benefits of the new index type are realized only if you create a new index or re-index an old one. Dates are now signed, and the max index key size has increased slightly from 819 to 1024 bytes. All operations that create a new index will result in a 2.0 index by default. For example: Upgrading a Sharded Cluster Changes Compact Command Concurrency Improvements Default Stack Size Index Performance Enhancements • Reindexing results on an older-version index results in a 2.0 index. However, reindexing on a secondary does not work in versions prior to 2.0. Do not reindex on a secondary. For a workaround, see SERVER-3866$^{884}$. $^{883}$https://jira.mongodb.org/browse/SERVER-2563 $^{884}$https://jira.mongodb.org/browse/SERVER-3866 Chapter 12. Release Notes 910 • Updates by _id • Removes • Long cursor iterations 1. Upgrade all config server instances first , in any order. Since config servers use two-phase commit, shard con- figuration metadata updates will halt until all are up and running. 2. Upgrade mongos routers in any order.