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

    https://github.com/apache/couchdb-couch/pull/185#discussion_r80059337
  
    --- 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),
    +    {ok, DataL} = file:pread(Fd, [{L, ?PREFIX_SIZE} || L <- Locations]),
    +    case locate_header(Fd, header_locations(Locations, DataL)) of
    +        {ok, _Location, HeaderBin} ->
    +            io:format("header at block ~p~n", [_Location div 
?SIZE_BLOCK]), % TEMP
    +            {ok, HeaderBin};
    +        _ ->
    +            ok = file:advise(
    +                Fd, hd(Locations), ReadCount * ?SIZE_BLOCK, dont_need),
    +            NextBlock = hd(Locations) div ?SIZE_BLOCK - 1,
    +            find_header_pread2(Fd, NextBlock, ReadCount)
    +    end.
    +
    +-spec read_count() -> non_neg_integer().
    +read_count() ->
    +    config:get_integer("couchdb", "find_header_read_count", 
?DEFAULT_READ_COUNT).
    +
    +-spec block_locations(block_id(), non_neg_integer()) -> [location()].
    +block_locations(Block, ReadCount) ->
    +    First = max(0, Block - ReadCount + 1),
    +    [?SIZE_BLOCK*B || B <- lists:seq(First, Block)].
    +
    +-spec header_locations([location()], [data()]) -> [{location(), 
header_size()}].
    +header_locations(Locations, DataL) ->
    +    lists:foldl(fun
    +        ({Loc, <<1, HeaderSize:32/integer>>}, Acc) ->
    +            [{Loc, HeaderSize} | Acc];
    +        (_, Acc) ->
    +            Acc
    +        end, [], lists:zip(Locations, DataL)).
    +
    +-spec locate_header(file:fd(), [{location(), header_size()}]) ->
    +    {ok, binary()} | not_found.
    +locate_header(_Fd, []) ->
    +    not_found;
    +locate_header(Fd, [{Location, Size}|LocationSizes]) ->
    +    case (catch load_header(Fd, Location, Size)) of
    +        {ok, HeaderBin} ->
    +            {ok, Location, HeaderBin};
    +        _Error ->
    +            locate_header(Fd, LocationSizes)
    +    end.
    +
    +-spec load_header(file:fd(), location(), header_size()) ->
    +    {ok, binary()} | no_return().
    +load_header(Fd, Location, Size) ->
    --- End diff --
    
    You should move this function up under load_header/2 and then rearrange 
load_header/2 so that it calls this function. Scrolling back and forth between 
the two trying to figure out the difference was quite confusing.


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