diff --git a/src/metainfo.rs b/src/metainfo.rs
index f48f9e9..ceb7b8d 100644
--- a/src/metainfo.rs
+++ b/src/metainfo.rs
@@ -107,6 +107,15 @@ pub struct MetainfoFile {
     pub path: PathBuf,
 }
 
+impl MetainfoFile {
+    pub fn new(length: u64, path: PathBuf) -> Self {
+        MetainfoFile {
+            length: length,
+            path: path,
+        }
+    }
+}
+
 fn sha1(bytes: &[u8]) -> Hash {
     let mut hasher = Sha1::new();
     hasher.update(bytes);
diff --git a/src/net/_session/disk.rs b/src/net/_session/disk.rs
new file mode 100644
index 0000000..f1af97b
--- /dev/null
+++ b/src/net/_session/disk.rs
@@ -0,0 +1,166 @@
+use std::cmp;
+use std::io::{self, Read, Seek, SeekFrom, Write};
+use std::sync::Arc;
+
+use net::_session::TorrentMap;
+use metainfo::{Hash, Metainfo};
+
+pub struct DiskManager<F> where F: Read + Seek + Write {
+    torrents: TorrentMap<Torrent<F>>,
+}
+
+impl<F> DiskManager<F> where F: Read + Seek + Write {
+    pub fn write_piece(&mut self, info_hash: &Hash, index: u32, piece: Vec<u8>) -> io::Result<()> {
+        let torrent = self.torrents.get_mut(&info_hash).unwrap();
+
+        let buffer_len = piece.len();
+        let mut buffer_pos: usize = 0;
+        // All the files in a torrent can be seen as a single contiguous file. These
+        // values are relative to this contiguous file.
+        let mut abs_pos = index as u64 * torrent.metainfo.piece_length as u64;
+        let mut abs_start = 0;
+        let mut abs_end = 0;
+
+        for (idx, file) in torrent.metainfo.files.iter().enumerate() {
+            abs_end += file.length;
+
+            if abs_start <= abs_pos && abs_pos < abs_end {
+                // The amount of bytes to write is the remaining number of bytes in the buffer,
+                // or the size of the file, whichever is smaller.
+                let remaining = (buffer_len - buffer_pos) as u64;
+                let write_len = cmp::min(remaining, abs_end - abs_pos) as usize;
+
+                // Seek in the file. The position given is relative to the start of the file.
+                torrent.files[idx].seek(SeekFrom::Start(abs_pos - abs_start))?;
+                torrent.files[idx].write_all(&piece[buffer_pos..buffer_pos + write_len])?;
+
+                abs_pos += write_len as u64;
+                buffer_pos += write_len;
+
+                // If the buffer position if equal to the length, we're done writing.
+                if buffer_pos == buffer_len {
+                    break
+                }
+            }
+
+            // The start of the next file is the end of this one.
+            abs_start = abs_end;
+        }
+
+        Ok(())
+    }
+}
+
+pub struct Torrent<F> where F: Read + Seek + Write {
+    metainfo: Arc<Metainfo>,
+    files: Vec<F>,
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    use std::io::{self, Read, Seek, SeekFrom, Write};
+    use std::path::PathBuf;
+    use std::sync::Arc;
+
+    use net::_session::TorrentMap;
+    use metainfo::{Hash, Metainfo, MetainfoFile};
+
+    struct MockFile {
+        seek: SeekFrom,
+        write_len: usize,
+    }
+
+    impl MockFile {
+        pub fn new() -> Self {
+            MockFile {
+                seek: SeekFrom::Start(0),
+                write_len: 0,
+            }
+        }
+    }
+
+    impl Read for MockFile {
+        fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> {
+            Ok(0)
+        }
+    }
+
+    impl Seek for MockFile {
+        fn seek(&mut self, pos: SeekFrom) -> io::Result<u64> {
+            println!("{:?}", pos);
+            self.seek = pos;
+            Ok(0)
+        }
+    }
+
+    impl Write for MockFile {
+        fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
+            self.write_len = buf.len();
+            Ok(0)
+        }
+
+        fn write_all(&mut self, buf: &[u8]) -> io::Result<()> {
+            self.write_len = buf.len();
+            Ok(())
+        }
+
+        fn flush(&mut self) -> io::Result<()> {
+            Ok(())
+        }
+    }
+
+    #[test]
+    fn test_write_piece() {
+        let info_hash = Hash::new([1u8; 20]);
+
+        let metainfo = Metainfo {
+            announce: "".into(),
+            announce_list: vec![],
+            files: vec![MetainfoFile::new(12, PathBuf::new()),
+                        MetainfoFile::new(6, PathBuf::new()),
+                        MetainfoFile::new(2, PathBuf::new())],
+            info_hash: info_hash,
+            piece_length: 10,
+            num_pieces: 2,
+            pieces: vec![],
+            length: 20,
+        };
+
+        let mut torrents = TorrentMap::new();
+        torrents.insert(info_hash, Torrent {
+            metainfo: Arc::new(metainfo),
+            files: vec![MockFile::new(), MockFile::new(), MockFile::new()],
+        });
+
+        let mut dm = DiskManager {
+            torrents: torrents,
+        };
+
+        {
+            // Write a piece in a single file that is larger than the piece.
+            dm.write_piece(&info_hash, 0, vec![2u8; 10]).unwrap();
+            let torrent = &dm.torrents[&info_hash];
+
+            assert_eq!(torrent.files[0].seek, SeekFrom::Start(0));
+            assert_eq!(torrent.files[0].write_len, 10);
+        }
+
+
+        {
+            // Write a piece over multiple files that are smaller that a piece.
+            dm.write_piece(&info_hash, 1, vec![2u8; 10]).unwrap();
+            let torrent = &dm.torrents[&info_hash];
+
+            assert_eq!(torrent.files[0].seek, SeekFrom::Start(10));
+            assert_eq!(torrent.files[0].write_len, 2);
+
+            assert_eq!(torrent.files[1].seek, SeekFrom::Start(0));
+            assert_eq!(torrent.files[1].write_len, 6);
+
+            assert_eq!(torrent.files[2].seek, SeekFrom::Start(0));
+            assert_eq!(torrent.files[2].write_len, 2);
+        }
+    }
+}
diff --git a/src/net/_session/mod.rs b/src/net/_session/mod.rs
new file mode 100644
index 0000000..14c7334
--- /dev/null
+++ b/src/net/_session/mod.rs
@@ -0,0 +1,7 @@
+pub mod disk;
+
+use std::collections::HashMap;
+
+use metainfo::Hash;
+
+type TorrentMap<T> = HashMap<Hash, T>;
diff --git a/src/net/mod.rs b/src/net/mod.rs
index 00df1d7..e5067f4 100644
--- a/src/net/mod.rs
+++ b/src/net/mod.rs
@@ -2,3 +2,4 @@ pub mod bitfield;
 mod buffers;
 pub mod peer;
 pub mod session;
+pub mod _session;