diff --git a/src/shrink/copier.rs b/src/shrink/copier.rs
index 3df0042..9fca58a 100644
--- a/src/shrink/copier.rs
+++ b/src/shrink/copier.rs
@@ -1,14 +1,59 @@
 use anyhow::Result;
+use std::fs::OpenOptions;
+use std::io::{Seek, SeekFrom, Write, Read};
+use std::os::unix::fs::OpenOptionsExt;
 
 pub type Sector = u64;
 
+#[derive(Debug)]
 pub struct Region {
-    src: Sector,
-    dest: Sector,
-    len: Sector,
+    pub src: Sector,
+    pub dest: Sector,
+    pub len: Sector,
 }
 
-// FIXME: pass in
-pub fn copy(path: &str, regions: &Vec<Region>) -> Result<()> {
+
+fn copy_step<W>(file: &mut W, src_byte: u64, dest_byte: u64, len: usize) -> Result<()>
+where
+    W: Write + Seek + Read,
+{
+    let mut buf = vec![0; len];
+    file.seek(SeekFrom::Start(src_byte))?;
+    file.read_exact(&mut buf[0..])?;
+    file.seek(SeekFrom::Start(dest_byte))?;
+    file.write_all(&buf)?;
+    Ok(())
+}
+
+fn copy_region<W>(file: &mut W, r: &Region) -> Result<()>
+where
+    W: Write + Seek + Read,
+{
+    const MAX_BYTES: Sector = 1024 * 1024 * 64;
+
+    let src_bytes = r.src * 512;
+    let dest_bytes = r.dest * 512;
+    let len_bytes = r.len * 512;
+    let mut written = 0;
+    while written != len_bytes {
+        let step = u64::min(len_bytes - written, MAX_BYTES);
+        copy_step(file, src_bytes + written, dest_bytes + written, step as usize)?;
+        written += step;
+    }
+    Ok(())
+}
+
+pub fn copy(path: &str, regions: &Vec<Region>) -> Result<()> {
+    let mut input = OpenOptions::new()
+        .read(true)
+        .write(true)
+        //.custom_flags(libc::O_DIRECT)
+        .open(path)?;
+
+    for r in regions {
+        eprintln!("copying {:?}", r);
+        copy_region(&mut input, r)?;
+    }
+
     Ok(())
 }
diff --git a/src/shrink/toplevel.rs b/src/shrink/toplevel.rs
index 9aa800b..bddc448 100644
--- a/src/shrink/toplevel.rs
+++ b/src/shrink/toplevel.rs
@@ -19,6 +19,7 @@ struct Pass1 {
     /// High blocks are beyond the new, reduced end of the pool.  These
     /// will need to be moved.
     nr_high_blocks: u64,
+    block_size: Option<u64>,
 }
 
 impl Pass1 {
@@ -27,6 +28,7 @@ impl Pass1 {
             allocated_blocks: FixedBitSet::with_capacity(0),
             nr_blocks,
             nr_high_blocks: 0,
+            block_size: None,
         }
     }
 }
@@ -34,6 +36,7 @@ impl Pass1 {
 impl xml::MetadataVisitor for Pass1 {
     fn superblock_b(&mut self, sb: &xml::Superblock) -> Result<()> {
         self.allocated_blocks.grow(sb.nr_data_blocks as usize);
+        self.block_size = Some(sb.data_block_size as u64);
         Ok(())
     }
 
@@ -414,8 +417,17 @@ mod tests {
     }
 }
 
-fn build_copy_regions(remaps: &Vec<(BlockRange, BlockRange)>) -> Vec<Region> {
-    let rs = Vec::new();
+fn build_copy_regions(remaps: &Vec<(BlockRange, BlockRange)>, block_size: u64) -> Vec<Region> {
+    let mut rs = Vec::new();
+
+    for (from, to) in remaps {
+        rs.push(Region {
+            src: from.start * block_size,
+            dest: to.start * block_size,
+            len: range_len(&from) * block_size,
+        });
+    }
+
     rs
 }
 
@@ -467,10 +479,10 @@ pub fn shrink(input_path: &str, output_path: &str, data_path: &str, nr_blocks: u
     let remaps = build_remaps(above, free);
     eprintln!("remappings {:?}.", remaps);
 
-    let regions = build_copy_regions(&remaps);
-    eprint!("Copying data...");
-    copier::copy(data_path, &regions);
-    eprintln!("done.");
+    let regions = build_copy_regions(&remaps, pass1.block_size.unwrap() as u64);
+    //eprint!("Copying data...");
+    copier::copy(data_path, &regions)?;
+    //eprintln!("done.");
 
     let output = OpenOptions::new()
         .read(false)