fix: buffer vector payload writes

src/vector/storage.rs

@@ -213,6 +213,9 @@ impl MmapVectorStorage {
             self.write_header(&mut file)?;
         }
 
+        // Buffer the payload instead of issuing one system call per float.
+        let mut file = io::BufWriter::new(file);
+
         // Write vectors
         for (id, vector) in vectors {
             // Write vector ID

tests/vector_storage_batch.rs

@@ -0,0 +1,46 @@
+use codanna::vector::{MmapVectorStorage, SegmentOrdinal, VectorDimension, VectorId};
+use std::time::Instant;
+
+fn check_batches(count: u32) {
+    let dir = tempfile::tempdir().unwrap();
+    let segment = SegmentOrdinal::new(0);
+    let mut storage =
+        MmapVectorStorage::new(dir.path(), segment, VectorDimension::dimension_384()).unwrap();
+    let vectors: Vec<_> = (1..=count)
+        .map(|id| (VectorId::new(id).unwrap(), vec![id as f32; 384]))
+        .collect();
+    let batch: Vec<_> = vectors.iter().map(|(id, v)| (*id, v.as_slice())).collect();
+    let middle = batch.len() / 2;
+
+    let start = Instant::now();
+    storage.write_batch(&batch[..middle]).unwrap();
+    let first_write = start.elapsed();
+    assert_eq!(storage.read_all_vectors().unwrap(), vectors[..middle]);
+    let start = Instant::now();
+    storage.write_batch(&batch[middle..]).unwrap();
+    println!(
+        "{count} vectors, 384 dimensions: {:?} writing",
+        first_write + start.elapsed()
+    );
+
+    let mut reopened = MmapVectorStorage::open(dir.path(), segment).unwrap();
+    assert_eq!(reopened.vector_count(), count as usize);
+    assert_eq!(reopened.read_all_vectors().unwrap(), vectors);
+    assert_eq!(
+        std::fs::metadata(dir.path().join("segment_0.vec"))
+            .unwrap()
+            .len(),
+        16 + u64::from(count) * (4 + 384 * 4)
+    );
+}
+
+#[test]
+fn appended_batches_are_readable_after_reopen() {
+    check_batches(32);
+}
+
+#[test]
+#[ignore = "measures a large synthetic embedding batch; run explicitly for write throughput"]
+fn large_embedding_batch_roundtrips() {
+    check_batches(10_000);
+}