text: leveldb File format
  ===================
  
      <beginning_of_file>
      [data block 1]
      [data block 2]
      ...
      [data block N]
      [meta block 1]
      ...
      [meta block K]
      [metaindex block]
      [index block]
      [Footer]        (fixed size; starts at file_size - sizeof(Footer))
      <end_of_file>
  
  The file contains internal pointers.  Each such pointer is called
  a BlockHandle and contains the following information:
  
      offset:   varint64
      size:     varint64
  
  See [varints](https://developers.google.com/protocol-buffers/docs/encoding#varints)
  for an explanation of varint64 format.
  
  1.  The sequence of key/value pairs in the file are stored in sorted
  order and partitioned into a sequence of data blocks.  These blocks
  come one after another at the beginning of the file.  Each data block
  is formatted according to the code in `block_builder.cc`, and then
  optionally compressed.
  
  2. After the data blocks we store a bunch of meta blocks.  The
  supported meta block types are described below.  More meta block types
  may be added in the future.  Each meta block is again formatted using
  `block_builder.cc` and then optionally compressed.
  
  3. A "metaindex" block.  It contains one entry for every other meta
  block where the key is the name of the meta block and the value is a
  BlockHandle pointing to that meta block.
  
  4. An "index" block.  This block contains one entry per data block,
  where the key is a string >= last key in that data block and before
  the first key in the successive data block.  The value is the
  BlockHandle for the data block.
  
  5. At the very end of the file is a fixed length footer that contains
  the BlockHandle of the metaindex and index blocks as well as a magic number.
  
          metaindex_handle: char[p];     // Block handle for metaindex
          index_handle:     char[q];     // Block handle for index
          padding:          char[40-p-q];// zeroed bytes to make fixed length
                                         // (40==2*BlockHandle::kMaxEncodedLength)
          magic:            fixed64;     // == 0xdb4775248b80fb57 (little-endian)
  
  ## "filter" Meta Block
  
  If a `FilterPolicy` was specified when the database was opened, a
  filter block is stored in each table.  The "metaindex" block contains
  an entry that maps from `filter.<N>` to the BlockHandle for the filter
  block where `<N>` is the string returned by the filter policy's
  `Name()` method.
  
  The filter block stores a sequence of filters, where filter i contains
  the output of `FilterPolicy::CreateFilter()` on all keys that are stored
  in a block whose file offset falls within the range
  
      [ i*base ... (i+1)*base-1 ]
  
  Currently, "base" is 2KB.  So for example, if blocks X and Y start in
  the range `[ 0KB .. 2KB-1 ]`, all of the keys in X and Y will be
  converted to a filter by calling `FilterPolicy::CreateFilter()`, and the
  resulting filter will be stored as the first filter in the filter
  block.
  
  The filter block is formatted as follows:
  
      [filter 0]
      [filter 1]
      [filter 2]
      ...
      [filter N-1]
  
      [offset of filter 0]                  : 4 bytes
      [offset of filter 1]                  : 4 bytes
      [offset of filter 2]                  : 4 bytes
      ...
      [offset of filter N-1]                : 4 bytes
  
      [offset of beginning of offset array] : 4 bytes
      lg(base)                              : 1 byte
  
  The offset array at the end of the filter block allows efficient
  mapping from a data block offset to the corresponding filter.
  
  ## "stats" Meta Block
  
  This meta block contains a bunch of stats.  The key is the name
  of the statistic.  The value contains the statistic.
  
  TODO(postrelease): record following stats.
  
      data size
      index size
      key size (uncompressed)
      value size (uncompressed)
      number of entries
      number of data blocks
  