Cloud-based data management platforms often employ multitenant databases, where service providers achieve economies of scale by consolidating multiple tenants on shared servers. In such database systems, a key functionality for service providers is database migration, which is useful for dynamic provisioning, load balancing, and system maintenance. Practical migration solutions have several requirements, including high availability, low performance overhead, and selfmanagement. We present Slacker, an end-to-end database migration system at the middleware level satisfying these requirements. Slacker leverages off-the-shelf hot backup tools to achieve live migration with effectively zero down-time. Additionally, Slacker minimizes the performance impact of migrations on both the migrating tenant and collocated tenants by leveraging ‘migration slack’, or resources that can be used for migration without excessively impacting query latency. We apply a PID controller to this problem, a...