MASK_BLK_PER_READ
devoff = dbtob(blkpos & ~MASK_BLK_PER_READ) + offset_raw_read;
off = dbtob(blkpos & MASK_BLK_PER_READ),
blkpos = (blkpos & ~MASK_BLK_PER_READ) + BLK_PER_READ;