US Patent 9613046 Parallel optimized remote synchronization of active block storage

Updating a second cluster server that backs up a first cluster server includes retrieving a first metadata file from a first cluster server. The first metadata file includes a first ordered list of block identifiers for data blocks stored on a first plurality of block servers. The updating also includes retrieving a second metadata file from a second cluster server. The second metadata file includes a second ordered list of block identifiers for data blocks stored on a second plurality of block servers. The updating also includes comparing the first metadata file to the second metadata file to determine a difference list. The difference list includes block identifiers from the first ordered list that differ from block identifiers of the second ordered list. The updating also includes sending, to the first cluster server, a request for data blocks associated with the block identifiers from the difference list.