MIT6.828_HW10_Bigger file for xv6

xv6文件系统扩展

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#MIT6.828 #HW10 #Bigger file #XV6

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本文探讨了如何通过添加二级索引节点来扩大xv6文件系统的文件大小限制，从原有的140个扇区扩展至16523个扇区，详细介绍了修改过程中的关键步骤和技术细节。

MIT6.828_HW10_Bigger file for xv6

当前 xv6 文件大小限制在 140 个扇区。直接索引节点 12 个，以及一个一级索引节点，其指向一个 sector，可包含 512/4 = 128个扇区。即总和为12+128 = 140个扇区。我们需要为xv6的文件节点添加一个二级索引节点，其包含128个一级索引节点的地址，每个一级索引节点又包含了128数据扇区。最终一个文件大小将增加到 16523 个扇区，大约8.5M。

The format of an on-disk inode is defined by struct dinode in fs.h. You’re particularly interested in NDIRECT, NINDIRECT, MAXFILE, and the addrs[] element of struct dinode. xv6 标准 inode 如下图所示。
inode 结构
在读写文件时，会调用 bmap()(in fs.c)。在写时，bmap 分配存储文件所需的 blocks并且如果需要存储块地址，还会分配一个 indirect block。

bmap() 处理两种页号. The bn argument is a “logical block” – a block number relative to the start of the file. The block numbers in ip->addrs[], and the argument to bread(), are disk block numbers. You can view bmap() as mapping a file’s logical block numbers into disk block numbers.

实现过程

修改 param.h 文件, 将 #define FSSIZE 1000改为

#define FSSIZE       20000

Download big.c into your xv6 directory, add it to the UPROGS list, start up xv6, and run big. It creates as big a file as xv6 will let it, and reports the resulting size. It should say 140 sectors.
做完预处理工作后运行big，应该有以下输出。

$ big
.
wrote 140 sectors
done; ok

You’ll have to have only 11 direct blocks, rather than 12, to make room for your new doubly-indirect block。之所以这么 arrange，可以避免我们修改 Inode 结点 addr数组的个数，索引结点总数依然是NDIRECT + 1。不过后来发现，修改NDIRECT,程序会更加清晰。即相应需要修改两处

// 1
#define NDIRECT 11
#define NINDIRECT (BSIZE / sizeof(uint))
// DOUBLE INDIRECT
#define NDINDIRECT (NINDIRECT * NINDIRECT)
#define MAXFILE (NDIRECT + NINDIRECT + NDINDIRECT)

// 2
// in-memory copy of an inode
struct inode {
  ...
  uint addrs[NDIRECT+ 2];
};

// 3
// On-disk inode structure
struct dinode {
  short type;           // File type
  ...          // Size of file (bytes)
  uint addrs[NDIRECT + 2];   // Data block addresses
};

buf 结构体。

struct buf {
  int flags;
  uint dev;
  uint blockno;
  struct sleeplock lock;
  uint refcnt;
  struct buf *prev; // LRU cache list
  struct buf *next;
  struct buf *qnext; // disk queue
  uchar data[BSIZE];
};

最终实现代码并不难写出，仿照一级索引结点写法使用相应的函数调用。[Note]需要注意 brelse(bp)的时机。详细过程可以查看注释。

static uint
bmap(struct inode *ip, uint bn)
{
  uint addr, *a;
  struct buf *bp;
  struct buf *bp2;
  // 直接索引结点数目 bn= 0~10
  if(bn < NDIRECT){
  	// 创建直接索引
    if((addr = ip->addrs[bn]) == 0)
      ip->addrs[bn] = addr = balloc(ip->dev);
    return addr;
  }
  bn -= NDIRECT;
  
  // #define NINDIRECT (BSIZE / sizeof(uint))  BSIZE = 512
  if(bn < NINDIRECT){
  	// 一级索引
    // Load indirect block, allocating if necessary.
    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;
  // 二级索引
  if (bn < NDINDIRECT) {
  	// 根结点
    if((addr = ip->addrs[NDIRECT+1]) == 0)
		ip->addrs[NDIRECT+1] = addr = balloc(ip->dev);

	bp = bread(ip->dev, addr);
    // 指向一级索引
    a = (uint *)bp->data;
    if ((addr = a[bn/NINDIRECT]) == 0) {
       a[bn/NINDIRECT] = addr = balloc(ip->dev);
	   log_write(bp);
	}
	
	bp2 = bread(ip->dev, addr);
	// 二级页表
	a = (uint *)bp2->data;
	if ((addr = a[bn%NINDIRECT]) == 0) {
	  a[bn%NINDIRECT] = addr = balloc(ip->dev);
      log_write(bp2);
	}
	
	brelse(bp2);
	brelse(bp);
	return addr;
  }

  panic("bmap: out of range");
}