When background creating an index, creation of other indexes is blocked, as described in the docs: "Only one index build at a time is permitted per collection.". However, this has the side effect that index creation of other indexes fails even if the index already exists. This is a specific problem when called indirectly from an ORM such as mongoid which tries to create indexes for its models on initialization. As a result, a server (e.g.: a rails instance) that uses that ORM cannot be restarted while a background index build is in progress because the attempted index creation (even though the index already exists) fails.

The error message from the mongo driver:

/opt/ruby-enterprise-1.8.7-2010.01/lib/ruby/gems/1.8/gems/mongo-1.0.1/lib/../lib/mongo/collection.rb:406:in `create_index':Mongo::OperationFailure: Failed to create index {:key=>

, :unique=>false, :name=>"status_1", :ns=>"thatdb.messages"}

This error went away as soon as the background index creation was done.

The ideal fix would be to allow multiple indexes to be created at once. A short term fix would be to have ensureIndex return success if the index already exists, regardless of what else might be happening.