BMAP_CHUNK_BYTES
(void) memset(buf, set ? 0xff : 0x00, BMAP_CHUNK_BYTES);
if (write(fd, buf, BMAP_CHUNK_BYTES) != BMAP_CHUNK_BYTES)
ml = BMAP_CHUNK_BYTES;
off = BMAP_CHUNK_NO(bn) * BMAP_CHUNK_BYTES;
ml = BMAP_CHUNK_BYTES;