1. A method of managing a cache in a data storage system, the method comprising: creating metadata by a cache manager operating in the data storage system, the metadata associating multiple data objects hosted by the data storage system with (i) identifying information for extents of storage that back the respective data objects in the data storage system, (ii) respective ranges of cache pages that cache data of the respective data objects, and (iii) respective clean/dirty cache state information, the extents of storage provisioned from a set of internal volumes of the data storage system, each extent providing a logical range of multiple storage blocks from an internal volume, the clean/dirty state information provided at per-extent granularity and indicating, for each extent, whether any cache page backed by the extent is dirty, each of one or more of the data objects composed of storage extents provisioned from multiple internal volumes in a one-to-many manner wherein each such data object is supported by storage extents from multiple internal volumes, each of one or more of the internal volumes having storage extents provisioned to multiple data objects in a one-to-many manner wherein each such internal volume supplies storage extents to multiple data objects; upon a failure of the cache, accessing the metadata by the cache manager to identify, from the clean/dirty cache state information, a set of the data objects for which the clean/dirty cache state information identifies at least one dirty extent; and taking offline each of the identified set of data objects, each of the identified set of data objects taken offline composed of storage extents provisioned from greater than one of the set of internal volumes.