1. A method for providing distributed hot-spare storage in a storage cluster, the method comprising: detecting a failure of a storage server computer in a redundant storage cluster having N active storage server computers, wherein N is greater than or equal to three; and reforming the storage cluster as a redundant storage cluster having N−1 active storage server computers in response to detecting the failure by copying mirrored data of the data stored on the failed storage server computer from the remaining active storage server computers to a distributed hot-spare storage node comprising a portion of unutilized storage space distributed across the remaining active storage server computers, wherein copying the mirrored data comprises: creating exception tables for each of the remaining active storage server computers of the storage cluster, each exception table comprising positive tabs identifying areas of the associated remaining active storage server computer that need to be copied to the distributed hot-spare storage node and negative tabs identifying areas of the distributed hot-spare storage node that need to be migrated from the other remaining active storage server computers; and copying the data from the remaining active storage server computers to the distributed hot-spare storage node according to the contents of the exception tables.