Ceph

It would be usefull to have convenience functions that work in terms of offset+length instead of chunks

• int decode(const map<int, buffer>& chunks, unsigned offset, unsigned len, bufferlist *out);
  

    set<int> want_to_read = range_to_chunks(offset, len) // compute what chunks must be retrieved
    set<int> available = the up set
    set<int> minimum = minimum_to_decode(want_to_read, available);
    map<int, buffer> available_chunks = retrieve_chunks_from_osds(minimum);
    map<int, buffer> chunks = transcode(want_to_read, available_chunks); // repairs if necessary
    out = bufferptr(concat_chunks(chunks), offset - offset of the first chunk, len)
  

• set<int> minimum_to_decode(unsigned offset, unsigned len, const set<int> &available_chunks);

The lower level plugin interface is useful from the plugin point of view. But erasure coded pools need something that is higher level.