1. A computer program product for taking corrupt portions of cache offline during runtime, the computer program product comprising: one or more computer readable hardware storage media and program instructions stored on the one or more computer readable hardware storage media, the program instructions comprising: program instructions to receive a notification of a section of a cache to be taken offline, wherein the section includes one or more sets in one or more indexes of the cache; program instructions to associate an indication with each set of the one or more sets in a first index of the one or more indexes, wherein the indication marks the respective set as unusable for future operations, wherein each indication is accessible for reference when an operation requires selection of a set in the first index of the cache, and wherein upon associating the indication with each set of the one or more sets in the first index, each set of the one or more sets in the first index remains accessible to one or more processors; prior to associating the indication with each set of the one or more sets in the first index, program instructions to determine that a number of viable sets in the first index, not included in the section, exceeds a minimum number of sets threshold for the first index; program instructions to purge data from the one or more sets in the first index of the cache, wherein program instructions to purge the data from the one or more sets in the first index of each cache comprise program instructions to store the data to a higher level memory; program instructions to mark each set of the one or more sets in the first index as invalid; program instructions to determine that the section includes an additional one or more sets in a second index of the one or more indexes; program instructions to increment to the second index of the one or more indexes; program instructions to determine that a number of viable sets in the second index, not included in the section, does not exceed a minimum number of sets threshold for the second index; and program instructions to generate an error.