tweak random advise

src/ra_log_segment.erl

@@ -115,17 +115,23 @@ open(Filename, Options) ->
     end.
 
 process_file(true, Mode, Filename, Fd, Options) ->
+    AccessPattern = maps:get(access_pattern, Options, random),
+    if AccessPattern == random andalso
+       Mode == read ->
+           %% advise for random access for any data beyond our best guess
+           %% index size, readers could pass `max_count` in the config to
+           %% ensure this is correct
+           ConfMaxCount = maps:get(max_count, Options, ?SEGMENT_MAX_ENTRIES),
+           Offset = ?HEADER_SIZE + (ConfMaxCount * ?INDEX_RECORD_SIZE_V2),
+           ok = file:advise(Fd, Offset, 0, random);
+       true ->
+           ok
+    end,
     case read_header(Fd) of
         {ok, Version, MaxCount} ->
             MaxPending = maps:get(max_pending, Options, ?SEGMENT_MAX_PENDING),
             IndexRecordSize = index_record_size(Version),
             IndexSize = MaxCount * IndexRecordSize,
-            AccessPattern = maps:get(access_pattern, Options, random),
-            if AccessPattern == random ->
-                   ok = file:advise(Fd, 0, 0, random);
-               true ->
-                   ok
-            end,
             {NumIndexRecords, DataOffset, Range, Index} =
                 recover_index(Fd, Version, MaxCount),
             IndexOffset = ?HEADER_SIZE + NumIndexRecords * IndexRecordSize,