The OpenStack Swift object storage service achieves high availability and durability by replicating object data across multiple object servers. If one disk or object-server fails then its data is always available on other object servers. Swift is able to apply this principle to not just disks and object servers but also independent availability zones and regions. This enables the deployment of so-called Global Clusters which provide a single object namespace spanning multiple geographically dispersed data-centres, each offering independent local access to object data.

This talk will describe some of the mechanisms that Swift provides for configuring and optimising Global Clusters. We will briefly describe how Swift's consistent hashing Ring maps objects to object servers and how that mapping algorithm can be configured to distribute copies of each object across data-centres. We will show how Swift's read and write affinity settings can be used to optimise WAN traffic in a Global Cluster. Finally we will discuss some of the challenges we faced when implementing Global Cluster support for erasure coded objects, and how those were overcome by enhancements to the Ring and the erasure coding write path.