SMR: Updates from LSFMM/Vault

SMR: Updates from LSFMM/Vault

After a week of tallking about SMR and thinking about the issues
raised in the original document and some of the solutions that
have arisen from LSFMM and Vault, there are some updates that need
to be made.

- outer zone is "conventional media recording" (CMR)
- some drives have much more CMR than others
- SMR architecture is targetted at single drives: no RAID!
- SMR architecture is targetted at unparitioned drives: fs only!
- CMR region is large enough for journal
- clarified the hint to vendors for "hybrid" SMR drives
- implications of 256MB zones made clear
  - /.zones/ needs to be heirarchical
  - zone groups are required for management of locality
  - zone inodes and bitmaps need to be reclaimable
- Cleaner updated for zone group awareness
- Inode data fork reverse mapping btree architecture added
- potential concurrent allocation/write issue solution added

Signed-off-by: Dave Chinner <dchinner@redhat.com>