OXenstored: an efficient hierarchical and transactional database using functional programming with reference cell comparisons

We describe in this paper our implementation of the Xenstored service which is part of the Xen architecture. Xenstored maintains a hierarchical and transactional database, used for storing and managing configuration values. We demonstrate in this paper that mixing functional data-structures together with reference cell comparison, which is a limited form of pointer comparison, is: (i) safe; and (ii) efficient. This demonstration is based, first, on an axiomatization of operations on the tree-like structure we used to represent the Xenstored database. From this axiomatization, we then derive an efficient algorithm for coalescing concurrent transactions modifying that structure. Finally, we experimentally compare the performance of our implementation, that we called OXenstored, and the C implementation of the Xenstored service distributed with the Xen hypervisor sources: the results show that Oxenstored is much more efficient than its C counterpart. As a direct result of this work, OXenstored will be included in future releases of Xenserver, the virtualization product distributed by Citrix Systems, where it will replace the current implementation of the Xenstored service.