I think a read cache device gets set up like RAID0, interleaved reads rather than redundant data. Examples of auxiliary devices where you want redundancy: ther

I think a read cache device gets set up like RAID0, interleaved reads rather than redundant data. Examples of auxiliary devices where you want redundancy: there can be a write cache (the ZFS Intent Log, or ZIL). You can also dedicate a fast device for hot items like the deduplication tables, or a dedicated device for tiny files where data can be stored directly in the directory data, rather than allocating a separate data block.