The vast majority of state produced by a typical computer is
generated, consumed, then lost forever. We argue that a
computer system should instead provide the ability to recall any past
state that existed on the computer, and further, that it should be
able to provide the lineage of any byte in a current or past state.
We call a system with this ability an
In this paper we present Arnold, the first practical eidetic computing platform. Preliminary data from several weeks of continuous use on our workstations shows that Arnold's storage requirements for 4 or more years of usage can be satisfied by adding a 4 TB hard drive to the system. Further, the performance overhead on almost all workloads we measured was under 8%. We show that Arnold can reconstruct prior state and answer lineage queries, including backward queries (on what did this item depend?) and forward queries (what other state did this item affect?).