US Patent 8200921 Data synchronization for two data mirrors with sidefiles

A data synchronization method (for a redundant data storage arrangement in which there are at least a primary storage entity and mirroring first and second remote storage entities in communication therewith, respectively, writes to the first and second remote storage entities being tracked via respective first and second sidefiles, the first and second storage entities having different levels of write-currency relative to each other) may include: comparing the first sidefile with the second sidefile; and updating writes stored at the second remote storage entity based upon the comparison of the first and second sidefiles.