diff --git a/src/common/composite_file.rs b/src/common/composite_file.rs
new file mode 100644
index 000000000..bea35f9fa
--- /dev/null
+++ b/src/common/composite_file.rs
@@ -0,0 +1,159 @@
+use std::io::Write;
+use common::CountingWriter;
+use std::collections::HashMap;
+use schema::Field;
+use common::VInt;
+use std::io;
+use directory::ReadOnlySource;
+use common::BinarySerializable;
+
+pub struct CompositeWrite<W: Write> {
+    write: CountingWriter<W>,
+    offsets: HashMap<Field, usize>,
+}
+
+impl<W: Write> CompositeWrite<W> {
+    pub fn wrap(w: W) -> CompositeWrite<W> {
+        CompositeWrite {
+            write: CountingWriter::wrap(w),
+            offsets: HashMap::new(),
+        }
+    }
+
+    pub fn for_field(&mut self, field: Field) -> &mut CountingWriter<W> {
+        let offset = self.write.written_bytes();
+        assert!(!self.offsets.contains_key(&field));
+        self.offsets.insert(field, offset);
+        &mut self.write
+    }
+
+    pub fn close(&mut self) -> io::Result<()> {
+        let footer_offset = self.write.written_bytes();
+        VInt(self.offsets.len() as u64).serialize(&mut self.write)?;
+
+        let mut offset_fields: Vec<_> = self.offsets.iter()
+            .map(|(field, offset)| (offset, field))
+            .collect();
+
+        offset_fields.sort();
+
+        let mut prev_offset = 0;
+        for (offset, field) in offset_fields {
+            VInt( (offset -  prev_offset) as u64).serialize(&mut self.write)?;
+            field.serialize(&mut self.write)?;
+            prev_offset = *offset;
+        }
+
+        let footer_len = (self.write.written_bytes() - footer_offset) as u32;
+        footer_len.serialize(&mut self.write)?;
+        self.write.flush()?;
+        Ok(())
+    }
+}
+
+
+pub struct CompositeFile {
+    data: ReadOnlySource,
+    offsets_index: HashMap<Field, (usize, usize)>,
+}
+
+impl CompositeFile {
+    pub fn open(data: ReadOnlySource) -> io::Result<CompositeFile> {
+        let end = data.len();
+        let footer_len_data = data.slice(end - 4, end);
+        let footer_len = u32::deserialize(&mut footer_len_data.as_slice())? as usize;
+
+        let footer_start = end - 4 - footer_len;
+        let footer_data = data.slice(footer_start, footer_start + footer_len);
+        let mut footer_buffer = footer_data.as_slice();
+        let num_fields = VInt::deserialize(&mut footer_buffer)?.0 as usize;
+
+        let mut fields = vec!();
+        let mut offsets = vec!();
+
+        let mut field_index = HashMap::new();
+
+        let mut offset = 0;
+        for _ in 0..num_fields {
+            offset += VInt::deserialize(&mut footer_buffer)?.0 as usize;
+            let field = Field::deserialize(&mut footer_buffer)?;
+            offsets.push(offset);
+            fields.push(field);
+        }
+        offsets.push(footer_start);
+        for i in 0..num_fields {
+            let field = fields[i];
+            let start_offset = offsets[i];
+            let end_offset = offsets[i+1];
+            field_index.insert(field, (start_offset, end_offset));
+        }
+
+        Ok(CompositeFile {
+            data: data.slice(0, footer_start),
+            offsets_index: field_index,
+        })
+    }
+
+    pub fn open_read(&self, field: Field) -> Option<ReadOnlySource> {
+        self.offsets_index
+            .get(&field)
+            .map(|&(from, to)| {
+                self.data.slice(from, to)
+            })
+    }
+}
+
+
+#[cfg(test)]
+mod test {
+
+    use std::io::Write;
+    use super::{CompositeWrite, CompositeFile};
+    use directory::{RAMDirectory, Directory};
+    use schema::Field;
+    use common::VInt;
+    use common::BinarySerializable;
+    use std::path::Path;
+
+    #[test]
+    fn test_composite_file() {
+        let path = Path::new("test_path");
+        let mut directory = RAMDirectory::create();
+        {
+            let w = directory.open_write(path).unwrap();
+            let mut composite_write = CompositeWrite::wrap(w);
+            {
+                let mut write_0 = composite_write.for_field(Field(0u32));
+                VInt(32431123u64).serialize(&mut write_0).unwrap();
+                write_0.flush().unwrap();
+            }
+
+            {
+                let mut write_4 = composite_write.for_field(Field(4u32));
+                VInt(2).serialize(&mut write_4).unwrap();
+                write_4.flush().unwrap();
+            }
+            composite_write.close().unwrap();
+        }
+        {
+            let r = directory.open_read(path).unwrap();
+            let composite_file = CompositeFile::open(r).unwrap();
+            {
+                let file0 = composite_file.open_read(Field(0u32)).unwrap();
+                let mut file0_buf = file0.as_slice();
+                let payload_0 = VInt::deserialize(&mut file0_buf).unwrap().0;
+                assert_eq!(file0_buf.len(), 0);
+                assert_eq!(payload_0, 32431123u64);
+            }
+            {
+                let file4 = composite_file.open_read(Field(4u32)).unwrap();
+                let mut file4_buf = file4.as_slice();
+                let payload_4 = VInt::deserialize(&mut file4_buf).unwrap().0;
+                assert_eq!(file4_buf.len(), 0);
+                assert_eq!(payload_4, 2u64);
+            }
+        }
+
+    }
+
+}
\ No newline at end of file
diff --git a/src/common/mod.rs b/src/common/mod.rs
index 0af9d2417..e8c8763f1 100644
--- a/src/common/mod.rs
+++ b/src/common/mod.rs
@@ -2,8 +2,11 @@ mod serialize;
 mod timer;
 mod vint;
 mod counting_writer;
+mod composite_file;
 pub mod bitpacker;
 
+
+pub(crate) use self::composite_file::{CompositeWrite, CompositeFile};
 pub use self::serialize::BinarySerializable;
 pub use self::timer::Timing;
 pub use self::timer::TimerTree;
diff --git a/src/fastfield/mod.rs b/src/fastfield/mod.rs
index 8b47d3a0e..31b241388 100644
--- a/src/fastfield/mod.rs
+++ b/src/fastfield/mod.rs
@@ -94,7 +94,7 @@ mod tests {
         }
         let source = directory.open_read(&path).unwrap();
         {
-            assert_eq!(source.len(), 38 as usize);
+            assert_eq!(source.len(), 35 as usize);
         }
         {
             let fast_field_readers = FastFieldsReader::from_source(source).unwrap();
@@ -128,7 +128,7 @@ mod tests {
         }
         let source = directory.open_read(&path).unwrap();
         {
-            assert_eq!(source.len(), 63 as usize);
+            assert_eq!(source.len(), 60 as usize);
         }
         {
             let fast_field_readers = FastFieldsReader::from_source(source).unwrap();
@@ -164,7 +164,7 @@ mod tests {
         }
         let source = directory.open_read(&path).unwrap();
         {
-            assert_eq!(source.len(), 36 as usize);
+            assert_eq!(source.len(), 33 as usize);
         }
         {
             let fast_field_readers = FastFieldsReader::from_source(source).unwrap();
@@ -197,7 +197,7 @@ mod tests {
         }
         let source = directory.open_read(&path).unwrap();
         {
-            assert_eq!(source.len(), 80044 as usize);
+            assert_eq!(source.len(), 80041 as usize);
         }
         {
             let fast_field_readers = FastFieldsReader::from_source(source).unwrap();
@@ -233,7 +233,7 @@ mod tests {
         }
         let source = directory.open_read(&path).unwrap();
         {
-            assert_eq!(source.len(), 17711 as usize);
+            assert_eq!(source.len(), 17708 as usize);
         }
         {
             let fast_field_readers = FastFieldsReader::from_source(source).unwrap();
diff --git a/src/fastfield/reader.rs b/src/fastfield/reader.rs
index aae1dd797..2ec8f66fc 100644
--- a/src/fastfield/reader.rs
+++ b/src/fastfield/reader.rs
@@ -1,6 +1,6 @@
 use std::io;
-use std::collections::HashMap;
 use directory::ReadOnlySource;
+use common::CompositeFile;
 use common::BinarySerializable;
 use DocId;
 use schema::{Field, SchemaBuilder};
@@ -240,8 +240,7 @@ impl FastFieldReader for I64FastFieldReader {
 /// It contains a mapping that associated these fields to
 /// the proper slice in the fastfield reader file.
 pub struct FastFieldsReader {
-    source: ReadOnlySource,
-    field_offsets: HashMap<Field, (u32, u32)>,
+    composite_file: CompositeFile,
 }
 
 impl FastFieldsReader {
@@ -251,31 +250,9 @@ impl FastFieldsReader {
     /// the list of the offset is read (as a footer of the
     /// data file).
     pub fn from_source(source: ReadOnlySource) -> io::Result<FastFieldsReader> {
-        let header_offset;
-        let field_offsets: Vec<(Field, u32)>;
-        {
-            let buffer = source.as_slice();
-            {
-                let mut cursor = buffer;
-                header_offset = u32::deserialize(&mut cursor)?;
-            }
-            {
-                let mut cursor = &buffer[header_offset as usize..];
-                field_offsets = Vec::deserialize(&mut cursor)?;
-            }
-        }
-        let mut end_offsets: Vec<u32> = field_offsets.iter().map(|&(_, offset)| offset).collect();
-        end_offsets.push(header_offset);
-        let mut field_offsets_map: HashMap<Field, (u32, u32)> = HashMap::new();
-        for (field_start_offsets, stop_offset) in
-            field_offsets.iter().zip(end_offsets.iter().skip(1)) {
-            let (field, start_offset) = *field_start_offsets;
-            field_offsets_map.insert(field, (start_offset, *stop_offset));
-        }
         Ok(FastFieldsReader {
-               field_offsets: field_offsets_map,
-               source: source,
-           })
+            composite_file: CompositeFile::open(source)?,
+        })
     }
 
     /// Returns the u64 fast value reader if the field
@@ -287,11 +264,8 @@ impl FastFieldsReader {
     /// # Panics
     /// May panic if the index is corrupted.
     pub fn open_reader<FFReader: FastFieldReader>(&self, field: Field) -> Option<FFReader> {
-        self.field_offsets
-            .get(&field)
-            .map(|&(start, stop)| {
-                     let field_source = self.source.slice(start as usize, stop as usize);
-                     FFReader::open(field_source)
-                 })
+        self.composite_file
+            .open_read(field)
+            .map(FFReader::open)
     }
 }
diff --git a/src/fastfield/serializer.rs b/src/fastfield/serializer.rs
index ef6ffedf9..590aee84a 100644
--- a/src/fastfield/serializer.rs
+++ b/src/fastfield/serializer.rs
@@ -3,7 +3,8 @@ use directory::WritePtr;
 use schema::Field;
 use common::bitpacker::{compute_num_bits, BitPacker};
 use common::CountingWriter;
-use std::io::{self, Write, Seek, SeekFrom};
+use common::CompositeWrite;
+use std::io::{self, Write};
 
 /// `FastFieldSerializer` is in charge of serializing
 /// fastfields on disk.
@@ -26,27 +27,17 @@ use std::io::{self, Write, Seek, SeekFrom};
 /// * `close_field()`
 /// * `close()`
 pub struct FastFieldSerializer {
-    write: CountingWriter<WritePtr>,
-    fields: Vec<(Field, u32)>,
-    min_value: u64,
-    field_open: bool,
-    bit_packer: BitPacker,
+    composite_write: CompositeWrite<WritePtr>,
 }
 
-
 impl FastFieldSerializer {
     /// Constructor
     pub fn new(write: WritePtr) -> io::Result<FastFieldSerializer> {
         // just making room for the pointer to header.
-        let mut counting_writer = CountingWriter::wrap(write);
-        0u32.serialize(&mut counting_writer)?;
+        let composite_write = CompositeWrite::wrap(write);
         Ok(FastFieldSerializer {
-               write: counting_writer,
-               fields: Vec::new(),
-               min_value: 0,
-               field_open: false,
-               bit_packer: BitPacker::new(0),
-           })
+            composite_write: composite_write,
+        })
     }
 
     /// Start serializing a new u64 fast field
@@ -54,23 +45,48 @@ impl FastFieldSerializer {
                               field: Field,
                               min_value: u64,
                               max_value: u64)
-                              -> io::Result<()> {
-        if self.field_open {
-            return Err(io::Error::new(io::ErrorKind::Other, "Previous field not closed"));
-        }
-        self.min_value = min_value;
-        self.field_open = true;
-        self.fields.push((field, self.write.written_bytes() as u32));
-        let write = &mut self.write;
+                              -> io::Result<FastSingleFieldSerializer<CountingWriter<WritePtr>>> {
+        let field_write = self
+            .composite_write
+            .for_field(field);
+        FastSingleFieldSerializer::open(
+            field_write,
+            min_value,
+            max_value)
+    }
+
+
+    /// Closes the serializer
+    ///
+    /// After this call the data must be persistently save on disk.
+    pub fn close(mut self) -> io::Result<()> {
+        self.composite_write.close()
+    }
+}
+
+pub struct FastSingleFieldSerializer<'a, W: Write + 'a> {
+    bit_packer: BitPacker,
+    write: &'a mut W,
+    min_value: u64,
+}
+
+impl<'a, W: Write> FastSingleFieldSerializer<'a, W> {
+
+    fn open(write: &'a mut W,
+           min_value: u64,
+           max_value: u64) -> io::Result<FastSingleFieldSerializer<'a, W>> {
         min_value.serialize(write)?;
         let amplitude = max_value - min_value;
         amplitude.serialize(write)?;
         let num_bits = compute_num_bits(amplitude);
-        self.bit_packer = BitPacker::new(num_bits as usize);
-        Ok(())
+        let bit_packer = BitPacker::new(num_bits as usize);
+        Ok(FastSingleFieldSerializer {
+            write: write,
+            bit_packer: bit_packer,
+            min_value: min_value,
+        })
     }
 
-
     /// Pushes a new value to the currently open u64 fast field.
     pub fn add_val(&mut self, val: u64) -> io::Result<()> {
         let val_to_write: u64 = val - self.min_value;
@@ -78,33 +94,7 @@ impl FastFieldSerializer {
         Ok(())
     }
 
-    /// Close the u64 fast field.
-    pub fn close_field(&mut self) -> io::Result<()> {
-        if !self.field_open {
-            return Err(io::Error::new(io::ErrorKind::Other, "Current field is already closed"));
-        }
-        self.field_open = false;
-        // adding some padding to make sure we
-        // can read the last elements with our u64
-        // cursor
-        self.bit_packer.close(&mut self.write)?;
-        Ok(())
-    }
-
-
-    /// Closes the serializer
-    ///
-    /// After this call the data must be persistently save on disk.
-    pub fn close(self) -> io::Result<usize> {
-        if self.field_open {
-            return Err(io::Error::new(io::ErrorKind::Other, "Last field not closed"));
-        }
-        let header_offset: usize = self.write.written_bytes() as usize;
-        let (mut write, written_size) = self.write.finish()?;
-        self.fields.serialize(&mut write)?;
-        write.seek(SeekFrom::Start(0))?;
-        (header_offset as u32).serialize(&mut write)?;
-        write.flush()?;
-        Ok(written_size)
+    pub fn close_field(mut self) -> io::Result<()> {
+        self.bit_packer.close(&mut self.write)
     }
 }
diff --git a/src/fastfield/writer.rs b/src/fastfield/writer.rs
index 52b29972f..1427a7b36 100644
--- a/src/fastfield/writer.rs
+++ b/src/fastfield/writer.rs
@@ -208,13 +208,14 @@ impl IntFastFieldWriter {
             (self.val_min, self.val_max)
         };
 
-        serializer.new_u64_fast_field(self.field, min, max)?;
+
+        let mut single_field_serializer = serializer.new_u64_fast_field(self.field, min, max)?;
 
         let mut cursor = self.vals.as_slice();
         while let Ok(VInt(val)) = VInt::deserialize(&mut cursor) {
-            serializer.add_val(val)?;
+            single_field_serializer.add_val(val)?;
         }
 
-        serializer.close_field()
+        single_field_serializer.close_field()
     }
 }
diff --git a/src/indexer/merger.rs b/src/indexer/merger.rs
index 468d867e7..f150f831a 100644
--- a/src/indexer/merger.rs
+++ b/src/indexer/merger.rs
@@ -175,18 +175,19 @@ impl IndexMerger {
 
             assert!(min_val <= max_val);
 
-            fast_field_serializer
+
+            let mut fast_single_field_serializer = fast_field_serializer
                 .new_u64_fast_field(field, min_val, max_val)?;
             for (max_doc, u64_reader, delete_bitset) in u64_readers {
                 for doc_id in 0..max_doc {
                     if !delete_bitset.is_deleted(doc_id) {
                         let val = u64_reader.get(doc_id);
-                        fast_field_serializer.add_val(val)?;
+                        fast_single_field_serializer.add_val(val)?;
                     }
                 }
             }
 
-            fast_field_serializer.close_field()?;
+            fast_single_field_serializer.close_field()?;
         }
         Ok(())
     }