Based on your statement I asked in the (excellent, friendly) prosody chat room. So, that mod_mam works for 0.9 because (from the mod_mam documentation) 'If no archive-capable storage backend can be opened then an in-memory one will be used as fallback.'

As far as I understand this means that a service restart (or a crash) wipes your archive and I assume you won't be able to migrate your archive between versions either?