Various systems and methods cache extent information for use in off-host backups. One method involves detecting that a first set of one or more blocks of client data has not been relocated subsequent to a prior off-host backup operation while also detecting that a second set of one or more blocks of the client data has been relocated subsequent to the prior off-host backup operation. In response to detecting that the first set has not been relocated, the method provides a media server with a previously calculated set of mapping information for the first set instead of recalculating that mapping information. In contrast, in response to detecting that the second set has been relocated subsequent to the prior off-host backup, the method recalculates mapping information for the second set. The media server is provided with the recalculated mapping information instead of previously calculated mapping information for the second set.