+1 for this, and it should be simple to implement since the actual compression  is decided based on the msgr2 connection feature negotiation. If we have an alternative we can just say that future OSDs will not accept this compression during connection feature negotiation.

One more point - we should be very careful not to compress the data in the buffer list too soon (and please excuse my ignorance about the exact data layering implementation in Ceph) - some feature have to look into the data (dedup...), so we need to make sure that we don't compress above the level that these features need the data.