diff --git a/src/chunkers.rs b/src/chunkers.rs
index 240393a..5286e03 100644
--- a/src/chunkers.rs
+++ b/src/chunkers.rs
@@ -1,8 +1,9 @@
-use anyhow::{anyhow, Context, Result};
+use anyhow::{anyhow, ensure, Context, Result};
 use io::prelude::*;
 use std::fs::File;
 use std::fs::OpenOptions;
 use std::io;
+use std::io::Take;
 use std::ops::Range;
 use std::os::unix::fs::FileExt;
 use std::path::Path;
@@ -21,20 +22,21 @@ pub enum Chunk {
 }
 
 pub struct ThickChunker {
-    input: File,
+    input: Take<File>,
     input_size: u64,
     total_read: u64,
-    block_size: usize,
+    block_size: u64,
 }
 
 impl ThickChunker {
-    pub fn new(input_path: &Path, block_size: usize) -> Result<Self> {
+    pub fn new(input_path: &Path, block_size: u64) -> Result<Self> {
         let input_size = thinp::file_utils::file_size(input_path)?;
         let input = OpenOptions::new()
             .read(true)
             .write(false)
             .open(input_path)
-            .context("couldn't open input file/dev")?;
+            .context("couldn't open input file/dev")?
+            .take(0);
 
         Ok(Self {
             input,
@@ -44,9 +46,11 @@ impl ThickChunker {
         })
     }
 
-    // FIXME: stop reallocating and zeroing these buffers
-    fn do_read(&mut self, mut buffer: Vec<u8>) -> Result<Option<Chunk>> {
-        self.input.read_exact(&mut buffer)?;
+    fn do_read(&mut self, size: u64) -> Result<Option<Chunk>> {
+        let mut buffer = Vec::with_capacity(size as usize);
+        self.input.set_limit(size);
+        let read_size = self.input.read_to_end(&mut buffer)?;
+        ensure!(read_size == size as usize, "short read");
         self.total_read += buffer.len() as u64;
         Ok(Some(Chunk::Mapped(buffer)))
     }
@@ -56,12 +60,10 @@ impl ThickChunker {
 
         if remaining == 0 {
             Ok(None)
-        } else if remaining >= self.block_size as u64 {
-            let buf = vec![0; self.block_size];
-            self.do_read(buf)
+        } else if remaining >= self.block_size {
+            self.do_read(self.block_size)
         } else {
-            let buf = vec![0; remaining as usize];
-            self.do_read(buf)
+            self.do_read(remaining)
         }
     }
 }
@@ -141,12 +143,7 @@ impl Iterator for ThinChunker {
     type Item = Result<Chunk>;
 
     fn next(&mut self) -> Option<Self::Item> {
-        let mc = self.next_chunk();
-        match mc {
-            Err(e) => Some(Err(e)),
-            Ok(Some(c)) => Some(Ok(c)),
-            Ok(None) => None,
-        }
+        self.next_chunk().transpose()
     }
 }
 
@@ -223,12 +220,7 @@ impl Iterator for DeltaChunker {
     type Item = Result<Chunk>;
 
     fn next(&mut self) -> Option<Self::Item> {
-        let mc = self.next_chunk();
-        match mc {
-            Err(e) => Some(Err(e)),
-            Ok(Some(c)) => Some(Ok(c)),
-            Ok(None) => None,
-        }
+        self.next_chunk().transpose()
     }
 }
 
diff --git a/src/content_sensitive_splitter.rs b/src/content_sensitive_splitter.rs
index c07d448..a2e6d1e 100644
--- a/src/content_sensitive_splitter.rs
+++ b/src/content_sensitive_splitter.rs
@@ -136,36 +136,41 @@ impl ContentSensitiveSplitter {
         let mut offset = 0;
         let mut remainder = self.unconsumed_len as usize;
         let min_size = self.window_size as usize / 4;
+        let max_size = self.window_size as usize * 8;
         let ws = self.window_size as usize;
 
+        if remainder < min_size {
+            offset += min_size - remainder;
+            remainder = min_size;
+        }
         while offset < data.len() {
-            let end = data.len();
-            if let Some(boundary) = self.hasher.next_match(&data[offset..end], self.mask_s) {
-                consumes.push(remainder + boundary);
-                offset += boundary;
-
-                let skip_size = std::cmp::min(data.len() - offset, min_size);
-                offset += skip_size;
-                remainder = skip_size;
-                continue;
-            } else {
-                offset += ws;
-                remainder += ws;
-            }
+            let len_s = ws - remainder;
+            if len_s > 0 {
+                let end = std::cmp::min(data.len(), offset + len_s);
+                if let Some(boundary) = self.hasher.next_match(&data[offset..end], self.mask_s) {
+                    consumes.push(remainder + boundary);
+                    offset += boundary + min_size;
+                    remainder = min_size;
+                    continue;
+                } else {
+                    offset += len_s;
+                    remainder += len_s;
+                }
 
-            if offset >= data.len() {
-                break;
+                if offset >= data.len() {
+                    break;
+                }
             }
-
-            if let Some(boundary) = self.hasher.next_match(&data[offset..], self.mask_l) {
+            let len_l = max_size - remainder;
+            let end = std::cmp::min(data.len(), offset + len_l);
+            if let Some(boundary) = self.hasher.next_match(&data[offset..end], self.mask_l) {
                 consumes.push(remainder + boundary);
-                offset += boundary;
-
-                let skip_size = std::cmp::min(data.len() - offset, min_size);
-                offset += skip_size;
-                remainder = skip_size;
+                offset += boundary + min_size;
+                remainder = min_size;
             } else {
-                break;
+                consumes.push(end - offset + remainder);
+                offset = end + min_size;
+                remainder = min_size;
             }
         }
 
diff --git a/src/slab.rs b/src/slab.rs
index 249467c..d8349b0 100644
--- a/src/slab.rs
+++ b/src/slab.rs
@@ -119,7 +119,7 @@ impl SlabOffsets {
 // derived data, and can be rebuilt with the repair fn.
 //
 // file := <header> <slab>*
-// header := <magic nr> <slab format version>
+// header := <magic nr> <slab format version> <flags>
 // slab := <magic nr> <len> <checksum> <compressed data>
 
 const FILE_MAGIC: u64 = 0xb927f96a6b611180;
@@ -196,7 +196,6 @@ fn writer_(shared: Arc<Mutex<SlabShared>>, rx: Receiver<SlabData>) -> Result<()>
         let buf = rx.recv();
         if buf.is_err() {
             // all send ends have been closed, so we're done.
-            assert!(queued.is_empty());
             break;
         }