Online Migration for Geo-distributed Storage Systems.

We consider the problem of migrating user data between data centers. We introduce distributed storage overlays, a simple abstraction that represents data as stacked layers in different places. Overlays can be readily used to cache data objects, migrate these caches, and migrate the home of data objects.We implement overlays as part of a key-value object store called Nomad, designed to span many data centers. Using Nomad, we compare overlays against commonmigration approaches and show that overlays are more flexible and impose less overhead. To drive migration decisions, we propose policies for predicting the location of future accesses, focusing on a web mail application.We evaluate themigration policies using real traces of user activity from Hotmail.