var nf int // Bytes written in fragment
		dataStart, dataEnd := sw.sp[0].Offset, sw.sp[0].endOffset()
		if sw.pos < dataStart { // In a hole fragment
			bf := b[:min(int64(len(b)), dataStart-sw.pos)]
			nf, err = zeroWriter{}.Write(bf)
		} else { // In a data fragment
			bf := b[:min(int64(len(b)), dataEnd-sw.pos)]
			nf, err = sw.fw.Write(bf)
		}
		b = b[nf:]
		sw.pos += int64(nf)
		if sw.pos >= dataEnd && len(sw.sp) > 1 {