hcrosse commented on code in PR #1537:
URL: 
https://github.com/apache/datafusion-ballista/pull/1537#discussion_r3028769478


##########
ballista/core/src/utils.rs:
##########
@@ -159,42 +159,74 @@ pub fn default_config_producer() -> SessionConfig {
     SessionConfig::new_with_ballista()
 }
 
-/// Stream data to disk in Arrow IPC format
+/// Stream data to disk in Arrow IPC format.
+///
+/// Batches are read from the async stream and forwarded through a bounded
+/// channel to a `spawn_blocking` task that performs all synchronous file I/O,
+/// keeping the tokio worker thread unblocked.
 pub async fn write_stream_to_disk(
     stream: &mut Pin<Box<dyn RecordBatchStream + Send>>,
     path: &str,
     disk_write_metric: &metrics::Time,
 ) -> Result<PartitionStats> {
-    let file = BufWriter::new(File::create(path).map_err(|e| {
-        error!("Failed to create partition file at {path}: {e:?}");
-        BallistaError::IoError(e)
-    })?);
+    let schema = stream.schema();
+    let path_owned = path.to_owned();
+    let write_metric = disk_write_metric.clone();
+
+    let (tx, mut rx) = tokio::sync::mpsc::channel::<RecordBatch>(2);
+
+    let handle = tokio::task::spawn_blocking(move || -> Result<()> {
+        let file = BufWriter::new(File::create(&path_owned).map_err(|e| {
+            error!("Failed to create partition file at {path_owned}: {e:?}");
+            BallistaError::IoError(e)
+        })?);
+
+        let options = IpcWriteOptions::default()
+            .try_with_compression(Some(CompressionType::LZ4_FRAME))?;
+
+        let mut writer =
+            StreamWriter::try_new_with_options(file, schema.as_ref(), 
options)?;
+
+        while let Some(batch) = rx.blocking_recv() {
+            let timer = write_metric.timer();
+            writer.write(&batch)?;
+            timer.done();
+        }
+        let timer = write_metric.timer();
+        writer.finish()?;
+        timer.done();
+        Ok(())
+    });
 
     let mut num_rows = 0;
     let mut num_batches = 0;
     let mut num_bytes = 0;
 
-    let options = IpcWriteOptions::default()
-        .try_with_compression(Some(CompressionType::LZ4_FRAME))?;
-
-    let mut writer =
-        StreamWriter::try_new_with_options(file, stream.schema().as_ref(), 
options)?;
-
-    while let Some(result) = stream.next().await {
-        let batch = result?;
+    let stream_err = loop {
+        match stream.next().await {
+            Some(Ok(batch)) => {
+                num_batches += 1;
+                num_rows += batch.num_rows();
+                num_bytes += batch.get_array_memory_size();

Review Comment:
   Good catch! Fixed to use the bytes on disk.



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