1. A method comprising: in a data storage system, assigning a reliability value to each of a plurality of logical containers of data; for each of the plurality of logical containers of data, allocating a block of physical storage devices to store data, the block of physical storage devices selected from a specific parity group of a plurality of parity groups of physical storage devices, wherein the specific parity group for a given block of physical storage devices is selected according to the reliability value associated with a logical container of data corresponding to the given block of physical storage devices; and assigning a specific one of a plurality of parity-group reliability values to each of the plurality of parity groups, wherein a specific parity-group reliability value assigned to a given parity group is based on a corresponding reliability value associated with one or more blocks of physical storage devices in the given parity group.