  In this paper we study the problem of storing reliably an archive of
versioned data. Specifically, we focus on systems where the differences
(deltas) between subsequent versions rather than the whole objects are stored -
a typical model for storing versioned data. For reliability, we propose erasure
encoding techniques that exploit the sparsity of information in the deltas
while storing them reliably in a distributed back-end storage system, resulting
in improved I/O read performance to retrieve the whole versioned archive. Along
with the basic techniques, we propose a few optimization heuristics, and
evaluate the techniques' efficacy analytically and with numerical simulations.
