diff --git a/core/io/mod.rs b/core/io/mod.rs
index ab299ef64..f7766ef84 100644
--- a/core/io/mod.rs
+++ b/core/io/mod.rs
@@ -24,21 +24,36 @@ pub trait File: Send + Sync {
         buffers: Vec<Arc<RefCell<Buffer>>>,
         c: Completion,
     ) -> Result<Completion> {
-        // FIXME: for now, stupid default so i dont have to impl for all backends
-        let counter = Rc::new(Cell::new(0));
-        let len = buffers.len();
+        use std::sync::atomic::{AtomicUsize, Ordering};
+        if buffers.is_empty() {
+            c.complete(0);
+            return Ok(c);
+        }
+        // naive default implementation can be overridden on backends where it makes sense to
         let mut pos = pos;
+        let outstanding = Arc::new(AtomicUsize::new(buffers.len()));
+        let total_written = Arc::new(AtomicUsize::new(0));
+
         for buf in buffers {
-            let _counter = counter.clone();
-            let _c = c.clone();
-            let default_c = Completion::new_write(move |_| {
-                _counter.set(_counter.get() + 1);
-                if _counter.get() == len {
-                    _c.complete(len as i32); // complete the original completion
-                }
-            });
             let len = buf.borrow().len();
-            self.pwrite(pos, buf, default_c)?;
+            let child_c = {
+                let c_main = c.clone();
+                let outstanding = outstanding.clone();
+                let total_written = total_written.clone();
+                Completion::new_write(move |n| {
+                    // accumulate bytes actually reported by the backend
+                    total_written.fetch_add(n as usize, Ordering::Relaxed);
+                    if outstanding.fetch_sub(1, Ordering::AcqRel) == 1 {
+                        // last one finished
+                        c_main.complete(total_written.load(Ordering::Acquire) as i32);
+                    }
+                })
+            };
+            if let Err(e) = self.pwrite(pos, buf.clone(), child_c) {
+                // best-effort: mark as done so caller won't wait forever
+                c.complete(-1);
+                return Err(e);
+            }
             pos += len;
         }
         Ok(c)