* lsm_btree
  * [2i] secondary index support
    * atomic multi-commit/recovery
    * add checkpoint/1 and sync/1 - flush pending writes to stable storage
      (nursery:finish() and finish/flush any merges)
  * [config] add config parameters on open
    * {sync, boolean()} fdsync or not on write
    * {cache, bytes(), name} share max(bytes) cache named 'name' via etc
  * [stats] statistics
    * For each level {#merges, {merge-time-min, max, average}}
  * [expiry] support for time based expiry, merge should eliminate expired data
  * add @doc strings and and -spec's
  * check to make sure every error returns with a reason {error, Reason}
  * lager; check for uses of lager:error/2
  * add version 1, crc to the files
  * add compression via snappy (https://github.com/fdmanana/snappy-erlang-nif)
  * add encryption
  * adaptive nursery sizing
  * add truncate/1 - quickly truncates a database to 0 items
  * count/1 - return number of items currently in tree
  * backpressure on fold operations
    - The "sync_fold" creates a snapshot (hard link to btree files), which
      provides consistent behavior but may use a lot of disk space if there is
      a lot of insertion going on.
    - The "async_fold" folds a limited number, and remembers the last key
      serviced, then picks up from there again. So you could see intermittent
      puts in a subsequent batch of results.


PHASE 2:
* lsm_btree
  * Define a standard struct which is the metadata added at the end of the
    file, e.g. [btree-nodes] [meta-data] [offset of meta-data]. This is written
    in lsm_btree_writer:flush_nodes, and read in lsm_btree_reader:open2.
  * [feature] compression, encryption on disk



REVIEW LITERATURE AND OTHER SIMILAR IMPLEMENTATAIONS:
* nessdb https://code.google.com/p/nessdb/source/browse/LSM-BTREE?r=3a1df166a19505a2369dd954e8fc6d0a545f3d7b
* http://tokutek.com/downloads/mysqluc-2010-fractal-trees.pdf page 14+
* http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.44.2782&rep=rep1&type=pdf


1: make the "first level" have more thatn 2^5 entries (controlled by the constant TOP_LEVEL in lsm_btree.hrl); this means a new set of files is opened/closed/merged for every 32 insert/updates/deletes. Setting this higher will just make the nursery correspondingly larger, which should be absolutely fine.

2: Right now, the streaming btree writer emits a btree page based on number of elements. This could be changed to be based on the size of the node (say, some block-size boudary) and then add padding at the end so that each node read becomes a clean block transfer. Right now, we're probably taking way to many reads.

3: Also, there is no caching of read nodes. So every time a btree node is visited it is also read from disk and term_to_binary'ed. But we need a caching system for that to work well (https://github.com/cliffmoon/cherly is difficult to build), it needs to be rebar-ified.

4: Also, the format for btree nodes could probably be optimized. Right now it's just binary_to_term of a key/value list as far as I remember. Perhaps we dont have to deserialize the entire thing.

5: It might also be good to employ a scheduler (github.com/esl/jobs<http://github.com/esl/jobs>) for issuing merges; because I think that it can be a problem for the OS if there are too many merges going on at the same time.