Methods, systems, and apparatus, including computer programs encoded on computer storage media, for online rehydration. One of the methods includes intercepting a request issued by a virtual computing instance to a primary storage system, wherein the request identifies first data stored in the primary storage system; determining whether the first data has been rehydrated by a rehydration system; and in response to determining that at least a first portion of the first data has not been rehydrated by the rehydration system: submitting, to the rehydration system, a rehydration request that identifies the first portion of the first data that has not been rehydrated; obtaining, from the rehydration system, a response that includes a copy of the first portion of the first data, wherein the rehydration system has obtained the copy from a backup storage system; and responding to the request using the copy of the first portion of the first data.