@@ -172,19 +172,33 @@ pub fn canonical_walk(dir: &Path) -> Result<Vec<(String, PathBuf)>, io::Error> {
172 /// The resulting CID uses the `blake3-hashseq` codec (0x80).
173 pub fn compute_content_id(dir: &Path) -> Result<Cid, io::Error> {
174     use rayon::prelude::*;
175+    use std::cell::RefCell;
176+    use std::io::Read;
177+
178+    thread_local! {
179+        static SCRATCH: RefCell<Vec<u8>> = const { RefCell::new(Vec::new()) };
180+    }
181 
182     let walked = canonical_walk(dir)?;
183 
178-    // Hash files in parallel across rayon workers. Each worker hashes a
179-    // whole file serially via mmap; the inter-file parallelism already
180-    // saturates cores, so intra-file rayon fan-out would just add dispatch
181-    // overhead.
184+    // Hash files in parallel across rayon workers. Each worker reuses a
185+    // thread-local scratch buffer to read the whole file and hashes from
186+    // RAM; this avoids per-file mmap setup/teardown and keeps allocations
187+    // out of the hot path.
188     let entries: Vec<(String, iroh_blobs::Hash)> = walked
189         .into_par_iter()
190         .map(|(name, path)| {
185-            let mut hasher = blake3::Hasher::new();
186-            hasher.update_mmap(&path).map_err(io::Error::other)?;
187-            Ok((name, hasher.finalize().into()))
191+            SCRATCH.with(|buf| {
192+                let mut buf = buf.borrow_mut();
193+                buf.clear();
194+                let mut file = std::fs::File::open(&path)?;
195+                let meta = file.metadata()?;
196+                buf.reserve(meta.len() as usize);
197+                file.read_to_end(&mut buf)?;
198+                let mut hasher = blake3::Hasher::new();
199+                hasher.update(&buf);
200+                Ok::<(String, iroh_blobs::Hash), io::Error>((name, hasher.finalize().into()))
201+            })
202         })
203         .collect::<Result<_, io::Error>>()?;
204