fsync where it matters

pkg/dotc1z/manager/local/local.go

@@ -59,10 +59,36 @@ func (l *localManager) copyFileToTmp(ctx context.Context) error {
 		}
 		defer f.Close()
 
-		_, err = io.Copy(tmp, f)
+		// Get source file size for verification
+		sourceStat, err := f.Stat()
+		if err != nil {
+			return fmt.Errorf("failed to stat source file: %w", err)
+		}
+		expectedSize := sourceStat.Size()
+
+		written, err := io.Copy(tmp, f)
 		if err != nil {
 			return err
 		}
+
+		// CRITICAL: Sync to ensure all data is written before file is used.
+		// This is especially important on ZFS ARC where writes may be cached
+		// and reads can happen before buffers are flushed to disk.
+		if err := tmp.Sync(); err != nil {
+			return fmt.Errorf("failed to sync temp file: %w", err)
+		}
+
+		// Verify file size matches what we wrote (defensive check)
+		stat, err := tmp.Stat()
+		if err != nil {
+			return fmt.Errorf("failed to stat temp file: %w", err)
+		}
+		if stat.Size() != written {
+			return fmt.Errorf("file size mismatch: wrote %d bytes but file is %d bytes", written, stat.Size())
+		}
+		if written != expectedSize {
+			return fmt.Errorf("copy size mismatch: expected %d bytes from source but copied %d bytes", expectedSize, written)
+		}
 	}
 
 	return nil
@@ -147,6 +173,12 @@ func (l *localManager) SaveC1Z(ctx context.Context) error {
 		return err
 	}
 
+	// CRITICAL: Sync to ensure data is written before function returns.
+	// This is especially important on ZFS ARC where writes may be cached.
+	if err := dstFile.Sync(); err != nil {
+		return fmt.Errorf("failed to sync destination file: %w", err)
+	}
+
 	log.Debug(
 		"successfully saved c1z locally",
 		zap.String("file_path", l.filePath),

pkg/dotc1z/manager/s3/s3.go

@@ -53,11 +53,30 @@ func (s *s3Manager) copyToTempFile(ctx context.Context, r io.Reader) error {
 	s.tmpFile = f.Name()
 
 	if r != nil {
-		_, err = io.Copy(f, r)
+		written, err := io.Copy(f, r)
 		if err != nil {
 			_ = f.Close()
 			return err
 		}
+
+		// CRITICAL: Sync to ensure all data is written before file is used.
+		// This is especially important on ZFS ARC where writes may be cached
+		// and reads can happen before buffers are flushed to disk.
+		if err := f.Sync(); err != nil {
+			_ = f.Close()
+			return fmt.Errorf("failed to sync temp file: %w", err)
+		}
+
+		// Verify file size matches what we wrote (defensive check)
+		stat, err := f.Stat()
+		if err != nil {
+			_ = f.Close()
+			return fmt.Errorf("failed to stat temp file: %w", err)
+		}
+		if stat.Size() != written {
+			_ = f.Close()
+			return fmt.Errorf("file size mismatch: wrote %d bytes but file is %d bytes", written, stat.Size())
+		}
 	}
 
 	return nil