Which works if you fail to read-only, and simply switch to reading from the non-failed region. More complicated is attempting to continue writing to a non-failed region and then attempting to bring everything consistent once the failed region is available again.

EDIT: If you were using hashing/uuids/guids for objects, this should be possible using a background task that'll scan various buckets in multiple regions and move objects when/where necessary to return to a consistent state.