diff --git a/examples/bag_info.rs b/examples/bag_info.rs
index a4a0aac..b757cfe 100644
--- a/examples/bag_info.rs
+++ b/examples/bag_info.rs
@@ -5,7 +5,7 @@ use regex::Regex;
 use ros_message::{MessagePath, Msg};
 use rsbag::{
     index::{BagIndex, ConnInfo},
-    reader::IoReader,
+    reader::{BagReader, MmapReader},
 };
 
 fn parse_msgdef(message_name: &str, msgdef: &str) -> rsbag::Result<Msg> {
@@ -53,28 +53,40 @@ fn main() {
 
     let bag_path = &args[1];
     let bag_file = File::open(bag_path).expect("Could not open bag file");
-    let mut bag_reader = IoReader::new(bag_file);
+    let mut bag_reader = MmapReader::new(bag_file).unwrap();
 
-    match BagIndex::read_all(&mut bag_reader) {
-        Ok(index) => {
-            for conn in &index.connections {
-                match parse_message_definitions(conn) {
-                    Ok(msgs) => {
-                        for msg in &msgs {
-                            info!(
-                                "message definition parsed: {:#?}",
-                                msg.fields()
-                                    .iter()
-                                    .filter(|field| !field.is_constant())
-                                    .map(ToString::to_string)
-                                    .collect::<Vec<_>>()
-                            );
-                        }
-                    }
-                    Err(err) => error!("could not parse message definition: {}", err),
+    let index = match BagIndex::read_all(&mut bag_reader) {
+        Ok(index) => index,
+        Err(err) => {
+            error!("bag parse error: {}", err);
+            return;
+        }
+    };
+
+    for conn in &index.connections {
+        match parse_message_definitions(conn) {
+            Ok(msgs) => {
+                for msg in &msgs {
+                    info!(
+                        "message definition parsed: {:#?}",
+                        msg.fields()
+                            .iter()
+                            .filter(|field| !field.is_constant())
+                            .map(ToString::to_string)
+                            .collect::<Vec<_>>()
+                    );
                 }
             }
+            Err(err) => error!("could not parse message definition: {}", err),
         }
-        Err(err) => error!("bag parse error: {}", err),
     }
+
+    let mut total_size = 0;
+
+    for chunk in &index.chunks {
+        let chunk_header = bag_reader.read_chunk_header(chunk.pos).unwrap();
+        total_size += chunk_header.uncompressed_size;
+    }
+
+    info!("total uncompressed size: {}", total_size);
 }
diff --git a/src/chunk.rs b/src/chunk.rs
new file mode 100644
index 0000000..c141226
--- /dev/null
+++ b/src/chunk.rs
@@ -0,0 +1,38 @@
+use std::str::FromStr;
+
+use crate::{parse::Header, Error, Result};
+
+#[derive(Clone, Copy, Debug)]
+pub enum Compression {
+    None,
+    Bz2,
+    Lz4,
+}
+
+impl FromStr for Compression {
+    type Err = Error;
+
+    fn from_str(s: &str) -> Result<Self, Self::Err> {
+        Ok(match s {
+            "none" => Compression::None,
+            "bz2" => Compression::Bz2,
+            "lz4" => Compression::Lz4,
+            _ => return Err(Error::UnsupportedCompression(s.to_string())),
+        })
+    }
+}
+
+#[derive(Debug)]
+pub struct ChunkHeader {
+    pub compression: Compression,
+    pub uncompressed_size: u32,
+}
+
+impl ChunkHeader {
+    pub fn from_header(header: Header) -> Result<Self> {
+        Ok(Self {
+            compression: header.read_string(b"compression")?.parse()?,
+            uncompressed_size: header.read_u32(b"size")?,
+        })
+    }
+}
diff --git a/src/error.rs b/src/error.rs
index ad7f55e..8fbb6b0 100644
--- a/src/error.rs
+++ b/src/error.rs
@@ -13,6 +13,8 @@ pub enum Error {
     UnsupportedVersion(parse::Version),
     #[error("unsupported encryptor: {0}")]
     UnsupportedEncryptor(String),
+    #[error("unsupported compression: {0}")]
+    UnsupportedCompression(String),
     #[error("unexpected EOF")]
     Eof,
     #[error("invalid header op: {0}")]
diff --git a/src/lib.rs b/src/lib.rs
index a9f05f5..50a6cb2 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -2,5 +2,6 @@ mod error;
 pub mod index;
 pub mod parse;
 pub mod reader;
+pub mod chunk;
 
 pub use error::{Error, Result};
diff --git a/src/reader.rs b/src/reader.rs
index ebc0065..5652fe1 100644
--- a/src/reader.rs
+++ b/src/reader.rs
@@ -2,6 +2,7 @@ use std::io::SeekFrom;
 
 use nom::number::streaming::le_u32;
 
+use crate::chunk::ChunkHeader;
 use crate::error::{Error, Result};
 use crate::index::{ChunkInfo, ConnInfo};
 use crate::parse::{self, Header, Op, Version};
@@ -59,4 +60,10 @@ pub trait BagReader {
         self.skip_data()?;
         ChunkInfo::from_header(header)
     }
+
+    fn read_chunk_header(&mut self, pos: u64) -> Result<ChunkHeader> {
+        self.seek(SeekFrom::Start(pos))?;
+        let header = self.read_header_op(Op::Chunk)?;
+        ChunkHeader::from_header(header)
+    }
 }