A method for data storage includes providing a mapping of data pages to physical pages, in which each physical page holds a non-integer number of the data pages, for storage of data in at least one memory block, including a plurality of the physical pages, in a memory device. The data pages that are mapped to the memory block are partitioned into groups, such that failure of any memory unit, which consists of a predefined number of the physical pages in the memory device, will produce errors in no more than one data page in each group. The data pages is stored in the physical pages of the memory block in accordance with the mapping, while a redundant storage scheme is applied among the data pages of each group.