1. A data storage system capable of presenting an image of a data store at a past time, the system comprising: a receiver for receiving a specification for a past time; an I/O processor which processes I/O requests directed to one or more target units of storage in a data store; an indexing module which records write requests processed by the I/O processor, the indexing module comprising a memory storing, for each write request, a record comprising: i) identification of target units of storage; ii) locations of data previously stored in the target units of storage; and iii) a write time denoting a time at which a write request was received; a mapping module that uses one or more records to generate a map of the locations of data stored in the target units of storage at the specified past time; and an image generation module that presents an image of the data store at the past time based, at least in part, on the map generated by the mapping module.