defer func() {
		for _, r := range readers {
			r.Close()
		}
	}()
	for i := range disks {
		r, w := io.Pipe()
		// Make sure we close the pipe so blocked writes doesn't stay around.
		defer r.CloseWithError(context.Canceled)

		readers[i] = newMetacacheReader(r)
		d := disks[i]

		// Send request to each disk.
		go func() {
			var werr error
			if d == nil {