操作系统原理作业（八）：bigger files for xv6

实验步骤

修改Makefile文件：将CPUS的值设为1，,并添加如下语句。目的是加快xv6创建大文件时的qemu的运行速度：

修改param.h文件，将文件系统的最大数据块数设置为20000：

下载big.c文件放入xv6的目录下，并在Makefile文件中的UPROGS列表中增加对big命令的引用：

启动xv6，输入big命令，显示“wrote 140 sectors”，说明所有的准备工作已完成：

修改fs.c中的bmap()函数如下：

static uint
bmap(struct inode *ip, uint bn)
{
  uint addr, *a, *indirect, *double_indirect,indirect_idx, double_indirect_idx;
  struct buf *bp, *bp2;
  if(bn < NDIRECT){
    if((addr = ip->addrs[bn]) == 0)
      ip->addrs[bn] = addr =balloc(ip->dev);
    return addr;
  }
  bn -= NDIRECT;
  if(bn < NINDIRECT){
    // Load indirect block, allocating ifnecessary.
    if((addr = ip->addrs[NDIRECT]) == 0)
      ip->addrs[NDIRECT] = addr =balloc(ip->dev);
    bp = bread(ip->dev, addr);
    a = (uint*)bp->data;
    if((addr = a[bn]) == 0){
      a[bn] = addr = balloc(ip->dev);
      log_write(bp);
    }
    brelse(bp);
    return addr;
  }
  bn -= NINDIRECT;
  if (bn < NINDIRECT*NINDIRECT) {
    // Load first indirect block, allocating ifnecessary.
    if((addr = ip->addrs[NDIRECT + 1]) == 0)
      ip->addrs[NDIRECT + 1] = addr =balloc(ip->dev);
    bp = bread(ip->dev, addr);
    indirect = (uint *) bp->data;
    indirect_idx = bn / NINDIRECT;
    if ((addr = indirect[indirect_idx]) == 0) {
      addr = indirect[indirect_idx] =balloc(ip->dev);
      log_write(bp);
    }
    bp2 = bread(ip->dev, addr);
    double_indirect = (uint *) bp2->data;
    double_indirect_idx = bn % NINDIRECT;
    if((addr = double_indirect[double_indirect_idx]) == 0) {
      addr =double_indirect[double_indirect_idx] = balloc(ip->dev);
      log_write(bp2);
    }
    brelse(bp2);
    brelse(bp);
    return addr;
  }
  panic("bmap: out of range");
}