Github user davisp commented on a diff in the pull request:

    https://github.com/apache/couchdb-couch/pull/185#discussion_r80057664
  
    --- Diff: src/couch_file.erl ---
    @@ -531,27 +537,96 @@ find_header(Fd, Block) ->
         {ok, Bin} ->
             {ok, Bin};
         _Error ->
    -        find_header(Fd, Block -1)
    +        find_header_pread2(Fd, Block -1)
         end.
     
     load_header(Fd, Block) ->
         {ok, <<1, HeaderLen:32/integer, RestBlock/binary>>} =
             file:pread(Fd, Block * ?SIZE_BLOCK, ?SIZE_BLOCK),
    -    TotalBytes = calculate_total_read_len(5, HeaderLen),
    -    case TotalBytes > byte_size(RestBlock) of
    +    TotalSize = total_read_size(5, HeaderLen),
    +    case TotalSize > byte_size(RestBlock) of
         false ->
    -        <<RawBin:TotalBytes/binary, _/binary>> = RestBlock;
    +        <<RawBin:TotalSize/binary, _/binary>> = RestBlock;
         true ->
             {ok, Missing} = file:pread(
                 Fd, (Block * ?SIZE_BLOCK) + 5 + byte_size(RestBlock),
    -            TotalBytes - byte_size(RestBlock)),
    +            TotalSize - byte_size(RestBlock)),
             RawBin = <<RestBlock/binary, Missing/binary>>
         end,
         <<Md5Sig:16/binary, HeaderBin/binary>> =
             iolist_to_binary(remove_block_prefixes(5, RawBin)),
         Md5Sig = couch_crypto:hash(md5, HeaderBin),
         {ok, HeaderBin}.
     
    +
    +%% Read a configurable number of block locations at a time using
    +%% file:pread/2.  At each block location, read ?PREFIX_SIZE (5) bytes;
    +%% if the first byte is <<1>>, assume it's a header block, and the
    +%% next 4 bytes hold the size of the header.
    +-spec find_header_pread2(file:fd(), block_id()) ->
    +    {ok, binary()} | no_valid_header.
    +find_header_pread2(Fd, Block) ->
    +    find_header_pread2(Fd, Block, read_count()).
    +
    +-spec find_header_pread2(file:fd(), block_id(), non_neg_integer()) ->
    +    {ok, binary()} | no_valid_header.
    +find_header_pread2(_Fd, Block, _ReadCount) when Block < 0 ->
    +    no_valid_header;
    +find_header_pread2(Fd, Block, ReadCount) ->
    +    Locations = block_locations(Block, ReadCount),
    --- End diff --
    
    block_locations is a trivial funciton that's only used once. You should 
inline it here so that we don't have to reference back and forth as the 
ordering is important so having that here would be more useful.


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