Ceph

Haomai and Dong proposed a vastly improved Transaction encoding during CDS. Video is here:

https://www.youtube.com/watch?v=8uYRDYdMywA&feature=youtu.be

Highlights:
- fixed-size struct for each op
- everything else is offset into a single bufferlist where variable-length bits accumulate
- very little data copying during encode/decode.

Comments from session:

http://pad.ceph.com/p/hammer-osd_transaction_encoding

 - use __le64, __le32 types
 - pointers directly into the bufferlist so there is no copy needed
 - need bufferlist method to ensure the next X bytes are contiguously allocated; reallocate (probably just) that section if necessary (#9954)