diff --git a/src/leveled_sft.erl b/src/leveled_sft.erl
index 9835fd0..c55e8ce 100644
--- a/src/leveled_sft.erl
+++ b/src/leveled_sft.erl
@@ -160,6 +160,7 @@
 -define(DWORD_SIZE, 8).
 -define(CURRENT_VERSION, {0,1}).
 -define(SLOT_COUNT, 256).
+-define(SLOT_GROUPWRITE_COUNT, 32).
 -define(BLOCK_SIZE, 32).
 -define(BLOCK_COUNT, 4).
 -define(FOOTERPOS_HEADERPOS, 2).
@@ -202,6 +203,72 @@ create_header(initial) ->
     CRC32 = erlang:crc32(H1),
 	<<H1/binary, CRC32:32/integer>>.
 
+%% Take a file handle at the sart position (after creating the header) and then
+%% write the Key lists to the file slot by slot.
+%%
+%% Slots are created then written in bulk to impove I/O efficiency.  Slots will
+%% be written in groups of 32
+
+
+write_group(Handle, KL1, KL2, SlotIndex, SerialisedSlots, Level, WriteFun) ->
+    write_group(Handle, KL1, KL2, {0, 0},
+                    SlotIndex, SerialisedSlots,
+                    {infinity, 0}, null, {last, null}, Level, WriteFun).
+
+
+write_group(Handle, KL1, KL2, {SlotCount, SlotTotal},
+                    SlotIndex, SerialisedSlots,
+                    {LSN, HSN}, LowKey, LastKey, Level, WriteFun)
+                    when SlotCount =:= ?SLOT_GROUPWRITE_COUNT ->
+    UpdHandle = WriteFun(slot , {Handle, SerialisedSlots}),
+    case maxslots_bylevel(SlotTotal, Level) of
+        reached ->
+            UpdHandle;
+        continue ->
+            write_group(UpdHandle, KL1, KL2, 0,
+                        SlotIndex, <<>>,
+                        {LSN, HSN}, LowKey, LastKey, Level, WriteFun)
+    end;
+write_group(Handle, KL1, KL2, {SlotCount, SlotTotal},
+                    SlotIndex, SerialisedSlots,
+                    {LSN, HSN}, LowKey, LastKey, Level, WriteFun) ->
+    SlotOutput = create_slot(KL1, KL2, Level),
+    {{LowKey_Slot, SegFilter, SerialisedSlot, LengthList},
+        {{LSN_Slot, HSN_Slot}, LastKey_Slot, Status},
+        KL1rem, KL2rem} = SlotOutput,
+    UpdSlotIndex = lists:append(SlotIndex,
+                                [{LowKey_Slot, SegFilter, LengthList}]),
+    UpdSlots = <<SerialisedSlots/binary, SerialisedSlot/binary>>,
+    SNExtremes = {min(LSN_Slot, LSN), max(HSN_Slot, HSN)},
+    FinalKey = case LastKey_Slot of null -> LastKey; _ -> LastKey_Slot end,
+    FirstKey = case LowKey of null -> LowKey_Slot; _ -> LowKey end,
+    case Status of
+        partial ->
+            UpdHandle = WriteFun(slot , {Handle, UpdSlots}),
+            WriteFun(finalise, {UpdHandle, UpdSlotIndex, SNExtremes,
+                                {FirstKey, FinalKey}});
+        full ->
+            write_group(Handle, KL1rem, KL2rem, {SlotCount + 1, SlotTotal + 1},
+                        UpdSlotIndex, UpdSlots,
+                        SNExtremes, FirstKey, FinalKey, Level, WriteFun)
+    end.
+        
+
+sftwrite_function(slot, {Handle, _SerialisedSlots}) ->
+    Handle;
+sftwrite_function(finalise,
+                    {Handle, _UpdSlotIndex, _SNExtremes, _KeyExtremes}) ->
+    Handle.
+
+maxslots_bylevel(SlotTotal, _Level) ->
+    case SlotTotal of
+        ?SLOT_COUNT ->
+            reached;
+        X when X < ?SLOT_COUNT ->
+            continue
+    end.
+
+
 %% Take two potentially overlapping lists of keys and output a Block,
 %% together with:
 %% - block status (full, partial)
@@ -230,7 +297,12 @@ create_block(KeyList1, KeyList2, Level) ->
 create_block(KeyList1, KeyList2,
                 BlockKeyList, {LSN, HSN}, SegmentList, _)
                                     when length(BlockKeyList)==?BLOCK_SIZE ->
-    {BlockKeyList, full, {LSN, HSN}, SegmentList, KeyList1, KeyList2};
+    case {KeyList1, KeyList2} of
+        {[], []} ->
+            {BlockKeyList, complete, {LSN, HSN}, SegmentList, [], []};
+        _ ->
+            {BlockKeyList, full, {LSN, HSN}, SegmentList, KeyList1, KeyList2}
+    end;
 create_block([], [],
                 BlockKeyList, {LSN, HSN}, SegmentList, _) ->
     {BlockKeyList, partial, {LSN, HSN}, SegmentList, [], []};
@@ -283,6 +355,11 @@ create_slot(KL1, KL2, _, _, SegLists, SerialisedSlot, LengthList,
     {{LowKey, generate_segment_filter(SegLists), SerialisedSlot, LengthList},
         {{LSN, HSN}, LastKey, partial},
         KL1, KL2};
+create_slot(KL1, KL2, _, _, SegLists, SerialisedSlot, LengthList,
+                                    {LowKey, LSN, HSN, LastKey, complete}) ->
+    {{LowKey, generate_segment_filter(SegLists), SerialisedSlot, LengthList},
+        {{LSN, HSN}, LastKey, partial},
+        KL1, KL2};
 create_slot(KL1, KL2, Level, BlockCount, SegLists, SerialisedSlot, LengthList,
                                     {LowKey, LSN, HSN, LastKey, _Status}) ->
     {BlockKeyList, Status,
@@ -304,10 +381,11 @@ create_slot(KL1, KL2, Level, BlockCount, SegLists, SerialisedSlot, LengthList,
                                     Status}
     end,
     SerialisedBlock = serialise_block(BlockKeyList),
+    % io:format("Serialised Block to be added ~w based on BlockKeyList ~w~n", [SerialisedBlock, BlockKeyList]),
     BlockLength = bit_size(SerialisedBlock),
     SerialisedSlot2 = <<SerialisedSlot/binary, SerialisedBlock/binary>>,
     create_slot(KL1b, KL2b, Level, BlockCount - 1, SegLists ++ [SegmentList],
-        SerialisedSlot2, LengthList ++ [BlockLength], TrackingMetadata).
+                SerialisedSlot2, LengthList ++ [BlockLength], TrackingMetadata).
 
 
 last([], {last, LastKey}) -> {keyonly, LastKey};
@@ -432,6 +510,8 @@ update_sequencenumbers({_, _, _, _}, LSN, HSN) ->
 %% This is more space efficient than the equivalent bloom filter and avoids
 %% the calculation of many hash functions.
 
+generate_segment_filter([SegL1]) ->
+    generate_segment_filter({SegL1, [], [], []});
 generate_segment_filter([SegL1, []]) ->
     generate_segment_filter({SegL1, [], [], []});
 generate_segment_filter([SegL1, SegL2, []]) ->
@@ -763,13 +843,16 @@ alternating_create_block_test() ->
                                                         1),
     BlockSize = length(MergedKeyList),
     ?assertMatch(BlockSize, 32),
-    ?assertMatch(ListStatus, full),
+    ?assertMatch(ListStatus, complete),
     K1 = lists:nth(1, MergedKeyList),
     ?assertMatch(K1, {{o, "Bucket1", "Key1"}, 1, {active, infinity}, null}),
     K11 = lists:nth(11, MergedKeyList),
     ?assertMatch(K11, {{o, "Bucket1", "Key9b"}, 1, {active, infinity}, null}),
     K32 = lists:nth(32, MergedKeyList),
-    ?assertMatch(K32, {{o, "Bucket4", "Key1"}, 1, {active, infinity}, null}).
+    ?assertMatch(K32, {{o, "Bucket4", "Key1"}, 1, {active, infinity}, null}),
+    HKey = {{o, "Bucket1", "Key0"}, 1, {active, infinity}, null},
+    {_, ListStatus2, _, _, _, _} = create_block([HKey|KeyList1], KeyList2, 1),
+    ?assertMatch(ListStatus2, full).
 
 
 merge_seglists_test() ->
@@ -886,3 +969,23 @@ createslot_stage3_test() ->
             [hash_for_segmentid({keyonly, {o, "BucketSeq", "Key00000004"}})],
             true),
     ?assertMatch(R3, {maybe_present, [0]}).
+
+
+
+testwrite_function(slot, {Handle, SerialisedSlots}) ->
+    lists:append(Handle, [SerialisedSlots]);
+testwrite_function(finalise, {Handle, UpdSlotIndex, SNExtremes, KeyExtremes}) ->
+    {Handle, UpdSlotIndex, SNExtremes, KeyExtremes}.
+
+writegroup_stage1_test() ->
+    {KL1, KL2} = sample_keylist(),
+    Output = write_group([], KL1, KL2,  [], <<>>, 1, fun testwrite_function/2),
+    {Handle, UpdSlotIndex, SNExtremes, KeyExtremes} = Output,
+    ?assertMatch(SNExtremes, {1,3}),
+    ?assertMatch(KeyExtremes, {{o, "Bucket1", "Key1"}, {o, "Bucket4", "Key1"}}),
+    [TopIndex|[]] = UpdSlotIndex,
+    {TopKey, _SegFilter,  LengthList} = TopIndex,
+    ?assertMatch(TopKey, {o, "Bucket1", "Key1"}),
+    TotalLength = lists:foldl(fun(X, Acc) -> Acc + X end, 0, LengthList),
+    ActualLength = lists:foldl(fun(X, Acc) -> Acc + bit_size(X) end, 0, Handle),
+    ?assertMatch(TotalLength, ActualLength).